io_uring/poll: flag request with REQ_F_POLL_TRIGGERED if it went through poll

If a request targeting a pollable file cannot get executed without
needing to wait for a POLLIN/POLLOUT trigger, then flag it with
REQ_F_POLL_TRIGGERED. Only supported for non-multishot requests, as it's
fully expected that multishot will always end up going through poll.

No functional changes in this patch, just in preparation for using this
information elsewhere.

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