io_uring: helper for empty req cache checks

Add io_req_cache_empty(), which checks if there are requests in the
inline req cache or not. It'll be needed in the future, but also nicely
cleans up a few spots poking into ->free_list directly.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/b18662389f3fb483d0bd07906647f65f6037475a.1649771823.git.asml.silence@gmail.com


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