Refactor adafs internal read/write funciton

 - Added adafs_pwrite and adafs_pread functions:
        they accept an OpenFile pointer instead of a fd so that can be
        called by function that already accessed the OpenFileMap.

 - Moved the writev function into internal adafs functions.

 - Added internal adafs_write function:
        instead messing up with the file position at the interception
        layer, we can use this internal function that hides the update of the
        file position two.