[dfsan] Remove hard-coded constant in release_shadow_space.c

compiler-rt/test/dfsan/release_shadow_space.c

int main(int argc, char **argv) {
   fprintf(
       stderr,
       "RSS at start: %zu, after mmap: %zu, after mmap+set label: %zu, after "
       "fixed map: %zu, after another mmap+set label: %zu, after munmap: %zu\n",
       before, after_mmap, after_mmap_and_set_label, after_fixed_mmap,
       after_mmap_and_set_label2, after_munmap);
 
   const size_t mmap_cost_kb = map_size >> 10;
-  const size_t mmap_shadow_cost_kb = 2 * mmap_cost_kb;
+  const size_t mmap_shadow_cost_kb = sizeof(dfsan_label) * mmap_cost_kb;
   assert(after_mmap >= before + mmap_cost_kb);
   assert(after_mmap_and_set_label >= after_mmap + mmap_shadow_cost_kb);
   assert(after_mmap_and_set_label2 >= before + mmap_cost_kb + mmap_shadow_cost_kb);
 
   // RSS may not change memory amount after munmap to the same level as the
   // start of the program. The assert checks the memory up to a delta.
   const size_t delta = 5000;
   assert(after_fixed_mmap <= before + delta);
   assert(after_munmap <= before + delta);
 
   return 0;
 }