io_uring: move up io_put_kbuf() and io_put_rw_kbuf()

Move them up to avoid explicit declaration. We will use them in later
patches.

Reviewed-by: Pavel Begunkov <asml.silence@gmail.com>
Signed-off-by: Hao Xu <haoxu@linux.alibaba.com>
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/3631243d6fc4a79bbba0cd62597fc8cd5be95924.1638714983.git.asml.silence@gmail.com


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