relocator: Protect grub_relocator_alloc_chunk_align() max_addr against integer underflow

This commit introduces integer underflow mitigation in max_addr calculation
in grub_relocator_alloc_chunk_align() invocation.

It consists of 2 fixes:
  1. Introduced grub_relocator_alloc_chunk_align_safe() wrapper function to perform
     sanity check for min/max and size values, and to make safe invocation of
     grub_relocator_alloc_chunk_align() with validated max_addr value. Replace all
     invocations such as grub_relocator_alloc_chunk_align(..., min_addr, max_addr - size, size, ...)
     by grub_relocator_alloc_chunk_align_safe(..., min_addr, max_addr, size, ...).
  2. Introduced UP_TO_TOP32(s) macro for the cases where max_addr is 32-bit top
     address (0xffffffff - size + 1) or similar.

Signed-off-by: Alexey Makhalov <amakhalov@vmware.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>

grub-core/lib/powerpc/relocator.c

@@ -115,10 +115,8 @@ grub_relocator32_boot (struct grub_relocator *rel,
   unsigned i;
   grub_relocator_chunk_t ch;
 
-  err = grub_relocator_alloc_chunk_align (rel, &ch, 0,
-					  (0xffffffff - stateset_size)
-					  + 1, stateset_size,
-					  sizeof (grub_uint32_t),
+  err = grub_relocator_alloc_chunk_align (rel, &ch, 0, UP_TO_TOP32 (stateset_size),
+					  stateset_size, sizeof (grub_uint32_t),
 					  GRUB_RELOCATOR_PREFERENCE_NONE, 0);
   if (err)
     return err;