mm: Assert that we preserve header vs region alignment

mm: Assert that we preserve header vs region alignment

grub_mm_region_init() does:

  h = (grub_mm_header_t) (r + 1);

where h is a grub_mm_header_t and r is a grub_mm_region_t.

Cells are supposed to be GRUB_MM_ALIGN aligned, but while grub_mm_dump
ensures this vs the region header, grub_mm_region_init() does not.

It's better to be explicit than implicit here: rather than changing
grub_mm_region_init() to ALIGN_UP(), require that the struct is
explicitly a multiple of the header size.

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