x86/kaslr: Drop redundant variable in __process_mem_region()

region.size can be trimmed to store the portion of the region before the
overlap, instead of a separate mem_vector variable.

Signed-off-by: Arvind Sankar <nivedita@alum.mit.edu>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/20200728225722.67457-9-nivedita@alum.mit.edu