malloc: Fix tst bug in malloc/tst-free-errno-malloc-hugetlb1.

When transparent hugepages (THP) are configured to 32MB on x86/loongarch
systems, the current big_size value may not be sufficiently large to
guarantee that free(ptr) [1] will call munmap(ptr_aligned, big_size).

Tested on x86_64 and loongarch64.

PS: Without this patch and using 32M THP, there is a about 50% chance
that malloc/tst-free-errno-malloc-hugetlb1 will fail on both x86_64 and
loongarch64.

[1] malloc/tst-free-errno.c:
...
       errno = 1789;
       /* This call to free() is supposed to call
            munmap (ptr_aligned, big_size);
          which increases the number of VMAs by 1, which is supposed
          to fail.  */
->     free (ptr);
       TEST_VERIFY (get_errno () == 1789);
     }
...

Reviewed-by: Wilco Dijkstra  <Wilco.Dijkstra@arm.com>

malloc/tst-free-errno.c

@@ -88,7 +88,7 @@ do_test (void)
 	FAIL_EXIT1 ("cannot create temporary file");
 
       /* Do a large memory allocation.  */
-      size_t big_size = 0x1000000;
+      size_t big_size = 0x3000000;
       void * volatile ptr = xmalloc (big_size - 0x100);
       char *ptr_aligned = (char *) ((uintptr_t) ptr & ~(pagesize - 1));
       /* This large memory allocation allocated a memory area