selftests/mm/mkdirty: fix memory leak in test_uffdio_copy()

Release memory before exception branch returns to prevent memory leaks

Checking tools/testing/selftests/mm/mkdirty.c ...
tools/testing/selftests/mm/mkdirty.c:283:3: error: Memory leak: src [memleak]
  return;
  ^

Link: https://lkml.kernel.org/r/20250114023838.48589-1-liuye@kylinos.cn
Signed-off-by: liuye <liuye@kylinos.cn>
Reviewed-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

tools/testing/selftests/mm/mkdirty.c

@@ -280,6 +280,7 @@ static void test_uffdio_copy(void)
 	dst = mmap(NULL, pagesize, PROT_READ, MAP_PRIVATE|MAP_ANON, -1, 0);
 	if (dst == MAP_FAILED) {
 		ksft_test_result_fail("mmap() failed\n");
+		free(src);
 		return;
 	}