io_uring: count CQEs in io_iopoll_check()

io_uring: count CQEs in io_iopoll_check()

A subsequent commit will allow uring_cmds that don't use iopoll on
IORING_SETUP_IOPOLL io_urings. As a result, CQEs can be posted without
setting the iopoll_completed flag for a request in iopoll_list or going
through task work. For example, a UBLK_U_IO_FETCH_IO_CMDS command could
call io_uring_mshot_cmd_post_cqe() to directly post a CQE. The
io_iopoll_check() loop currently only counts completions posted in
io_do_iopoll() when determining whether the min_events threshold has
been met. It also exits early if there are any existing CQEs before
polling, or if any CQEs are posted while running task work. CQEs posted
via io_uring_mshot_cmd_post_cqe() or other mechanisms won't be counted
against min_events.

Explicitly check the available CQEs in each io_iopoll_check() loop
iteration to account for CQEs posted in any fashion.

Signed-off-by: Caleb Sander Mateos <csander@purestorage.com>
Link: https://patch.msgid.link/20260302172914.2488599-4-csander@purestorage.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>