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

In case of an overlap, the beginning of the region should be used even
if it is exactly image_size, not just strictly larger.

Signed-off-by: Arvind Sankar <nivedita@alum.mit.edu>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/20200728225722.67457-6-nivedita@alum.mit.edu