ext4: fix miss unlock 'sb->s_umount' in extents_kunit_init()

There's warning as follows when do ext4 kunit test:
WARNING: kunit_try_catch/15923 still has locks held!
7.0.0-rc3-next-20260309-00028-g73f965a1bbb1-dirty #281 Tainted: G E N
1 lock held by kunit_try_catch/15923:
#0: ffff888139f860e0 (&type->s_umount_key#70/1){+.+.}-{4:4}, at: alloc_super.constprop.0+0x172/0xa90
Call Trace:
<TASK>
dump_stack_lvl+0x180/0x1b0
debug_check_no_locks_held+0xc8/0xd0
do_exit+0x1502/0x2b20
kthread+0x3a9/0x540
ret_from_fork+0xa76/0xdf0
ret_from_fork_asm+0x1a/0x30

As sget() will return 'sb' which holds 's->s_umount' lock. However,
"extents-test" miss unlock this lock.
So unlock 's->s_umount' in the end of extents_kunit_init().

Fixes: cb1e0c1d1fad ("ext4: kunit tests for extent splitting and conversion")
Signed-off-by: Ye Bin <yebin10@huawei.com>
Reviewed-by: Ritesh Harjani (IBM) <ritesh.list@gmail.com>
Reviewed-by: Ojaswin Mujoo <ojaswin@linux.ibm.com>
Link: https://patch.msgid.link/20260330133035.287842-2-yebin@huaweicloud.com
Signed-off-by: Theodore Ts'o <tytso@mit.edu>