selftests/mm: fix WARNING comparing pointer to 0

Remove comparing pointer to 0 to avoid this warning from coccinelle:

./tools/testing/selftests/mm/map_populate.c:80:16-17: WARNING comparing pointer to 0, suggest !E
./tools/testing/selftests/mm/map_populate.c:80:16-17: WARNING comparing pointer to 0

Link: https://lkml.kernel.org/r/20230817160033.90079-1-tuananhlfc@gmail.com
Signed-off-by: Anh Tuan Phan <tuananhlfc@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Author: tuananhlfc

tools/testing/selftests/mm/map_populate.c

@@ -77,7 +77,7 @@ int main(int argc, char **argv)
 	unsigned long *smap;
 
 	ftmp = tmpfile();
-	BUG_ON(ftmp == 0, "tmpfile()");
+	BUG_ON(!ftmp, "tmpfile()");
 
 	ret = ftruncate(fileno(ftmp), MMAP_SZ);
 	BUG_ON(ret, "ftruncate()");