mm: Adjust new region size to take management overhead into account

mm: Adjust new region size to take management overhead into account

When grub_memalign() encounters out-of-memory, it will try
grub_mm_add_region_fn() to request more memory from system firmware.
However, the size passed to it doesn't take region management overhead
into account. Adding a memory area of "size" bytes may result in a heap
region of less than "size" bytes really available. Thus, the new region
may not be adequate for current allocation request, confusing
out-of-memory handling code.

This patch introduces GRUB_MM_MGMT_OVERHEAD to address the region
management overhead (e.g. metadata, padding). The value of this new
constant must be large enough to make sure grub_memalign(align, size)
always succeeds after a successful call to
  grub_mm_init_region(addr, size + align + GRUB_MM_MGMT_OVERHEAD),
for any given addr and size (assuming no integer overflow).

The size passed to grub_mm_add_region_fn() is now correctly adjusted,
thus if grub_mm_add_region_fn() succeeded, current allocation request
can always succeed.

Signed-off-by: Zhang Boyang <zhangboyang.id@gmail.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>