nbd: Reclassify sockets to avoid lockdep circular dependency

nbd: Reclassify sockets to avoid lockdep circular dependency

syzbot reported a possible circular locking dependency in udp_sendmsg()
where fs_reclaim can be triggered while holding sk_lock, and fs_reclaim
can eventually depend on another sk_lock (e.g., if NBD is used for swap
or writeback and NBD uses TLS/TCP which acquires sk_lock).

Since the UDP socket and the NBD TCP/TLS socket are different, this is a
false positive. Fix this by reclassifying NBD sockets to a separate lock
class when they are added to the NBD device.

This is similar to what nvme-tcp and other network block devices do.

Fixes: ffa1e7ada456 ("block: Make request_queue lockdep splats show up earlier")
Reported-by: syzbot+607cdcf978b3e79da878@syzkaller.appspotmail.com
Closes: https://lore.kernel.org/netdev/6a2cdafe.428ffe26.258b27.0161.GAE@google.com/T/#u
Signed-off-by: Eric Dumazet <edumazet@google.com>
Link: https://patch.msgid.link/20260613042619.1108126-1-edumazet@google.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>