legacy_ion.h: a backwards-compatibility copy of ion.h

cloned from include/linux/ion.h at:

commit b6b0b653
Author: Brian Muramatsu <btmura@google.com>
Date:   Mon Jul 30 22:54:08 2012 -0700

    Add heap_mask to ion_allocation_data

Signed-off-by: Iliyan Malchev <malchev@google.com>