fix PTRDIFF_MAX check in the 32-bit mmap wrapper

It wasn't making use of the page-rounded size.

Change-Id: I911345fcfd6419a4dbc18a4622b42e1b76729abb

libc/bionic/mmap.cpp

@@ -48,7 +48,7 @@ void* mmap64(void* addr, size_t size, int prot, int flags, int fd, off64_t offse
 
   // prevent allocations large enough for `end - start` to overflow
   size_t rounded = BIONIC_ALIGN(size, PAGE_SIZE);
-  if (rounded < size || size > PTRDIFF_MAX) {
+  if (rounded < size || rounded > PTRDIFF_MAX) {
     errno = ENOMEM;
     return MAP_FAILED;
   }