ipv6: also use netdev_hold() in ip6_route_check_nh()

In blamed commit, we missed the fact that ip6_validate_gw()
could change dev under us from ip6_route_check_nh()

In this fix, I use GFP_ATOMIC in order to not pass too many additional
arguments to ip6_validate_gw() and ip6_route_check_nh() only
for a rarely used debug feature.

syzbot reported:

refcount_t: decrement hit 0; leaking memory.
WARNING: CPU: 0 PID: 5006 at lib/refcount.c:31 refcount_warn_saturate+0x1d7/0x1f0 lib/refcount.c:31
Modules linked in:
CPU: 0 PID: 5006 Comm: syz-executor403 Not tainted 6.4.0-rc5-syzkaller-01229-g97c5209b3d37 #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 05/27/2023
RIP: 0010:refcount_warn_saturate+0x1d7/0x1f0 lib/refcount.c:31
Code: 05 fb 8e 51 0a 01 e8 98 95 38 fd 0f 0b e9 d3 fe ff ff e8 ac d9 70 fd 48 c7 c7 00 d3 a6 8a c6 05 d8 8e 51 0a 01 e8 79 95 38 fd <0f> 0b e9 b4 fe ff ff 48 89 ef e8 1a d7 c3 fd e9 5c fe ff ff 0f 1f
RSP: 0018:ffffc900039df6b8 EFLAGS: 00010282
RAX: 0000000000000000 RBX: 0000000000000000 RCX: 0000000000000000
RDX: ffff888026d71dc0 RSI: ffffffff814c03b7 RDI: 0000000000000001
RBP: ffff888146a505fc R08: 0000000000000001 R09: 0000000000000000
R10: 0000000000000001 R11: 0000000000000001 R12: 1ffff9200073bedc
R13: 00000000ffffffef R14: ffff888146a505fc R15: ffff8880284eb5a8
FS: 0000555556c88300(0000) GS:ffff8880b9800000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00000000004585c0 CR3: 000000002b1b1000 CR4: 00000000003506f0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Call Trace:
<TASK>
__refcount_dec include/linux/refcount.h:344 [inline]
refcount_dec include/linux/refcount.h:359 [inline]
ref_tracker_free+0x539/0x820 lib/ref_tracker.c:236
netdev_tracker_free include/linux/netdevice.h:4097 [inline]
netdev_put include/linux/netdevice.h:4114 [inline]
netdev_put include/linux/netdevice.h:4110 [inline]
fib6_nh_init+0xb96/0x1bd0 net/ipv6/route.c:3624
ip6_route_info_create+0x10f3/0x1980 net/ipv6/route.c:3791
ip6_route_add+0x28/0x150 net/ipv6/route.c:3835
ipv6_route_ioctl+0x3fc/0x570 net/ipv6/route.c:4459
inet6_ioctl+0x246/0x290 net/ipv6/af_inet6.c:569
sock_do_ioctl+0xcc/0x230 net/socket.c:1189
sock_ioctl+0x1f8/0x680 net/socket.c:1306
vfs_ioctl fs/ioctl.c:51 [inline]
__do_sys_ioctl fs/ioctl.c:870 [inline]
__se_sys_ioctl fs/ioctl.c:856 [inline]
__x64_sys_ioctl+0x197/0x210 fs/ioctl.c:856
do_syscall_x64 arch/x86/entry/common.c:50 [inline]

Fixes: 70f7457ad6d6 ("net: create device lookup API with reference tracking")
Reported-by: syzbot <syzkaller@googlegroups.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Jakub Kicinski <kuba@kernel.org>
Cc: David Ahern <dsahern@kernel.org>
Reviewed-by: Jakub Kicinski <kuba@kernel.org>
Reviewed-by: David Ahern <dsahern@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>