nbd: don't warn when reclassifying a busy socket lock

nbd_reclassify_socket() warns via WARN_ON_ONCE() if the socket lock is
held at the point of reclassification. That assertion was copied from
nvme-tcp, where the socket is created internally by the kernel
(sock_create_kern()) and is never visible to user space, so the lock
is guaranteed to be free.

NBD is different: the socket is looked up from a user-supplied fd in
nbd_get_socket(), and user space retains that fd. A concurrent syscall
on the same socket (or softirq processing taking bh_lock_sock() on a
connected TCP socket) can legitimately hold the lock at the instant
NBD reclassifies it. sock_allow_reclassification() then returns false
and the WARN_ON_ONCE() fires, which turns into a crash under
panic_on_warn. This is reachable by simply racing NBD_CMD_CONNECT
against socket activity on the same fd, as reported by syzbot.

Hitting a held lock here is expected for an externally owned socket and
is not a kernel bug, so skip reclassification silently instead of
warning. Reclassification is a lockdep-only annotation, so skipping it
in the rare racing case is harmless.

Reported-by: syzbot+6b85d1e39a5b8ed9a954@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=6b85d1e39a5b8ed9a954
Fixes: d532cddb6c ("nbd: Reclassify sockets to avoid lockdep circular dependency")
Signed-off-by: Deepanshu Kartikey <kartikey406@gmail.com>
Acked-by: Eric Dumazet <edumazet@google.com>
Link: https://patch.msgid.link/20260621235255.66015-1-kartikey406@gmail.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

drivers/block/nbd.c

@@ -1246,7 +1246,7 @@ static void nbd_reclassify_socket(struct socket *sock)
 {
 	struct sock *sk = sock->sk;
 
-	if (WARN_ON_ONCE(!sock_allow_reclassification(sk)))
+	if (!sock_allow_reclassification(sk))
 		return;
 
 	switch (sk->sk_family) {