Reorganizing DMU Code

Reorganizing code in dmu.c so that the Direct IO patches will only
show the additions to dmu.c instead of also showing functions that
have been moved.

The code had to be reorganized a bit in order for the Direct IO code
paths in dmu.c to call some of the original DMU API.

Signed-off-by: Brian Atkinson <[email protected]>
Authored-by: Brian Atkinson <[email protected]>

Author: bwatkinson