lib: test_hmm: implement a device release method

Unloading the HMM test module produces the following warning:

[ 3782.224783] ------------[ cut here ]------------
[ 3782.226323] Device 'hmm_dmirror0' does not have a release() function, it is broken and must be fixed. See Documentation/core-api/kobject.rst.
[ 3782.230570] WARNING: drivers/base/core.c:2567 at device_release+0x185/0x210, CPU#20: rmmod/1924
[ 3782.233949] Modules linked in: test_hmm(-) nvidia_uvm(O) nvidia(O)
[ 3782.236321] CPU: 20 UID: 0 PID: 1924 Comm: rmmod Tainted: G O 7.0.0-rc1+ #374 PREEMPT(full)
[ 3782.240226] Tainted: [O]=OOT_MODULE
[ 3782.241639] Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS rel-1.17.0-0-gb52ca86e094d-prebuilt.qemu.org 04/01/2014
[ 3782.246193] RIP: 0010:device_release+0x185/0x210
[ 3782.247860] Code: 00 00 fc ff df 48 8d 7b 50 48 89 fa 48 c1 ea 03 80 3c 02 00 0f 85 86 00 00 00 48 8b 73 50 48 85 f6 74 11 48 8d 3d db 25 29 03 <67> 48 0f b9 3a e9 0d ff ff ff 48 b8 00 00 00 00 00 fc ff df 48 89
[ 3782.254211] RSP: 0018:ffff888126577d98 EFLAGS: 00010246
[ 3782.256054] RAX: dffffc0000000000 RBX: ffffffffc2b70310 RCX: ffffffff8fe61ba1
[ 3782.258512] RDX: 1ffffffff856e062 RSI: ffff88811341eea0 RDI: ffffffff91bbacb0
[ 3782.261041] RBP: ffff888111475000 R08: 0000000000000001 R09: fffffbfff856e069
[ 3782.263471] R10: ffffffffc2b7034b R11: 00000000ffffffff R12: 0000000000000000
[ 3782.265983] R13: dffffc0000000000 R14: ffff88811341eea0 R15: 0000000000000000
[ 3782.268443] FS: 00007fd5a3689040(0000) GS:ffff88842c8d0000(0000) knlGS:0000000000000000
[ 3782.271236] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[ 3782.273251] CR2: 00007fd5a36d2c10 CR3: 00000001242b8000 CR4: 00000000000006f0
[ 3782.275362] Call Trace:
[ 3782.276071] <TASK>
[ 3782.276678] kobject_put+0x146/0x270
[ 3782.277731] hmm_dmirror_exit+0x7a/0x130 [test_hmm]
[ 3782.279135] __do_sys_delete_module+0x341/0x510
[ 3782.280438] ? module_flags+0x300/0x300
[ 3782.281547] do_syscall_64+0x111/0x670
[ 3782.282620] entry_SYSCALL_64_after_hwframe+0x4b/0x53
[ 3782.284091] RIP: 0033:0x7fd5a3793b37
[ 3782.285303] Code: 73 01 c3 48 8b 0d c9 82 0c 00 f7 d8 64 89 01 48 83 c8 ff c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 44 00 00 b8 b0 00 00 00 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 8b 0d 99 82 0c 00 f7 d8 64 89 01 48
[ 3782.290708] RSP: 002b:00007ffd68b7dc68 EFLAGS: 00000206 ORIG_RAX: 00000000000000b0
[ 3782.292817] RAX: ffffffffffffffda RBX: 000055e3c0d1c770 RCX: 00007fd5a3793b37
[ 3782.294735] RDX: 0000000000000000 RSI: 0000000000000800 RDI: 000055e3c0d1c7d8
[ 3782.296661] RBP: 0000000000000000 R08: 1999999999999999 R09: 0000000000000000
[ 3782.298622] R10: 00007fd5a3806ac0 R11: 0000000000000206 R12: 00007ffd68b7deb0
[ 3782.300576] R13: 00007ffd68b7e781 R14: 000055e3c0d1b2a0 R15: 00007ffd68b7deb8
[ 3782.301963] </TASK>
[ 3782.302371] irq event stamp: 5019
[ 3782.302987] hardirqs last enabled at (5027): [<ffffffff8cf1f062>] __up_console_sem+0x52/0x60
[ 3782.304507] hardirqs last disabled at (5036): [<ffffffff8cf1f047>] __up_console_sem+0x37/0x60
[ 3782.306086] softirqs last enabled at (4940): [<ffffffff8cd9a4b0>] __irq_exit_rcu+0xc0/0xf0
[ 3782.307567] softirqs last disabled at (4929): [<ffffffff8cd9a4b0>] __irq_exit_rcu+0xc0/0xf0
[ 3782.309105] ---[ end trace 0000000000000000 ]---

This is because the test module doesn't have a device.release method. In
this case one probably isn't needed for correctness - the device structs
are in a static array so don't need freeing when the final reference goes
away.

However some device state is freed on exit, so to ensure this happens at
the right time and to silence the warning move the deinitialisation to a
release method and assign that as the device release callback. Whilst
here also fix a minor error handling bug where cdev_device_del() wasn't
being called if allocation failed.

Link: https://lore.kernel.org/20260331063445.3551404-4-apopple@nvidia.com
Fixes: 6a760f58c792 ("mm/hmm/test: use char dev with struct device to get device node")
Signed-off-by: Alistair Popple <apopple@nvidia.com>
Acked-by: Balbir Singh <balbirs@nvidia.com>
Tested-by: Zenghui Yu (Huawei) <zenghui.yu@linux.dev>
Cc: David Hildenbrand <david@kernel.org>
Cc: Jason Gunthorpe <jgg@ziepe.ca>
Cc: Leon Romanovsky <leon@kernel.org>
Cc: Liam Howlett <liam.howlett@oracle.com>
Cc: Lorenzo Stoakes (Oracle) <ljs@kernel.org>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Matthew Brost <matthew.brost@intel.com>
Cc: <stable@vger,kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>