vm: fix overflow in memblock do_alloc when allocating large aligned blocks

allocating a large power-of-2 block with memblock may cause the base pointer of a region to exceed the limit pointer after it has been aligned during the free region scan in do_alloc().

vm/memblock.c

@@ -252,7 +252,12 @@ static phys_addr_t do_alloc(size_t size, phys_addr_t align)
 			base += align;
 		}
 
-		size_t region_size = it.it_limit - base + 1;
+		size_t region_size = 0;
+
+		if (it.it_limit > base) {
+			region_size = it.it_limit - base + 1;
+		}
+
 		if (region_size >= size) {
 			allocated_base = it.it_base;
 			allocated_limit = base + size;