io_uring: move req_ref_get() and friends

Move all request refcount helpers to avoid forward declarations in the
future.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/89fd36f6f3fe5b733dfe4546c24725eee40df605.1628705069.git.asml.silence@gmail.com


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