io_uring: don't miss setting REQ_F_DOUBLE_POLL

When adding a second poll entry we should set REQ_F_DOUBLE_POLL
unconditionally. We might race with the first entry removal but that
doesn't change the rule.

Fixes: a18427bb2d9b ("io_uring: optimise submission side poll_refs")
Reported-and-tested-by:  <syzbot+49950ba66096b1f0209b@syzkaller.appspotmail.com>
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/8b680d83ded07424db83e8745585e7a6d72826ef.1657203020.git.asml.silence@gmail.com


Signed-off-by: Jens Axboe <axboe@kernel.dk>