x86/kaslr: Eliminate 'start_orig' local variable from __process_mem_region()

Set the region.size within the loop, which removes the need for
start_orig.

Signed-off-by: Arvind Sankar <nivedita@alum.mit.edu>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/20200728225722.67457-8-nivedita@alum.mit.edu