mm: When adding a region, merge with region after as well as before

mm: When adding a region, merge with region after as well as before

On x86_64-efi (at least) regions seem to be added from top down. The mm
code will merge a new region with an existing region that comes
immediately before the new region. This allows larger allocations to be
satisfied that would otherwise be the case.

On powerpc-ieee1275, however, regions are added from bottom up. So if
we add 3x 32MB regions, we can still only satisfy a 32MB allocation,
rather than the 96MB allocation we might otherwise be able to satisfy.

  * Define 'post_size' as being bytes lost to the end of an allocation
    due to being given weird sizes from firmware that are not multiples
    of GRUB_MM_ALIGN.

  * Allow merging of regions immediately _after_ existing regions, not
    just before. As with the other approach, we create an allocated
    block to represent the new space and the pass it to grub_free() to
    get the metadata right.

Signed-off-by: Daniel Axtens <dja@axtens.net>
Tested-by: Stefan Berger <stefanb@linux.ibm.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>
Tested-by: Patrick Steinhardt <ps@pks.im>