direct-io: unify argument passing by adding a dio_args structure

direct-io: unify argument passing by adding a dio_args structure

The O_DIRECT IO path is a mess of arguments. Clean that up by passing
those arguments in a dedicated dio_args structure.

This is in preparation for changing the internal implementation to be
page based instead of using iovecs.

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>