io_uring: reuse io_alloc_req()

Don't duplicate io_alloc_req() in io_req_caches_free() but reuse the
helper.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/6005fc88274864a49fc3096c22d8bdd605cf8576.1665891182.git.asml.silence@gmail.com


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