io_uring: add helper to return req to cache list

Don't hand code wq_stack_add_head() to ->free_list, which serves for
recycling io_kiocb, add a helper doing it for us.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/f206f575486a8dd3d52f074ab37ed146b2d215b7.1649771823.git.asml.silence@gmail.com


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