io_uring: inline io_free_req()

Inline io_free_req() into its only user and remove an underscore prefix
from __io_free_req().

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/ed114edef5c256a644f4839bb372df70d8df8e3f.1650056133.git.asml.silence@gmail.com


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