io_uring: inline io_flush_cached_reqs

io_flush_cached_reqs() isn't descriptive and has only one caller, inline
it into __io_alloc_req_refill().

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/ec38abe65a883d9fe6b169793119ce86806655a4.1649771823.git.asml.silence@gmail.com


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