x86/kaslr: Fix off-by-one error in process_gb_huge_pages()

If the remaining size of the region is exactly 1Gb, there is still one
hugepage that can be reserved.

Signed-off-by: Arvind Sankar <nivedita@alum.mit.edu>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/20200728225722.67457-11-nivedita@alum.mit.edu