x86/kaslr: Simplify process_gb_huge_pages()

Replace the loop to determine the number of 1Gb pages with arithmetic.

Signed-off-by: Arvind Sankar <nivedita@alum.mit.edu>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/20200728225722.67457-13-nivedita@alum.mit.edu