net: handle NULL ->poll gracefully

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: 5bdec0811f4d ("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>