io_uring: move small helpers to headers

There is a bunch of inline helpers that will be useful not only to the
core of io_uring, move them to headers.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/22df99c83723e44cba7e945e8519e64e3642c064.1655310733.git.asml.silence@gmail.com


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