[POWERPC] Fix alignment problem in rh_alloc_align() with exact-sized blocks

When an rheap is created, the caller can specify the alignment to use.  In
rh_alloc_align(), if a free block is found that is the exact size needed
(including extra space for alignment), that configured alignment value is not
used to align the pointer.  Instead, the default alignment is used.  If the
default alignment is smaller than the configured alignment, then the returned
value will not be aligned correctly.

Signed-off-by: Timur Tabi <timur@freescale.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>