net: handle NULL ->poll gracefully

The big aio poll revert broke various network protocols that don't
implement ->poll as a patch in the aio poll serie removed sock_no_poll
and made the common code handle this case.

Reported-by: syzbot+57727883dbad76db2ef0@syzkaller.appspotmail.com
Reported-by: syzbot+cdb0d3176b53d35ad454@syzkaller.appspotmail.com
Reported-by: syzbot+2c7e8f74f8b2571c87e8@syzkaller.appspotmail.com
Reported-by: Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>
Fixes: a11e1d432b51 ("Revert changes to convert to ->poll_mask() and aio IOCB_CMD_POLL")
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

net/socket.c

@@ -1133,6 +1133,8 @@ static __poll_t sock_poll(struct file *file, poll_table *wait)
 	__poll_t events = poll_requested_events(wait);
 
 	sock_poll_busy_loop(sock, events);
+	if (!sock->ops->poll)
+		return 0;
 	return sock->ops->poll(file, sock, wait) | sock_poll_busy_flag(sock);
 }