meminfo: Fix dmabufinfo total ref accounting

The number of references to a buffer total_refs was not accounting for
a process holding multiple references to the same buffer.

Also, clarify AppendDmaBufInfo functionality in comments

Change-Id: I044a18acad1570492a925d51b9464d5b0e60ca35
Signed-off-by: Erick Reyes <erickreyes@google.com>