selftest/vm: uninitialized variable in main()

Initialize "length" to zero by default.

Link: https://lkml.kernel.org/r/YtZzjvHXVXMXxpXO@kili
Fixes: ff712a6 ("selftests/vm: cleanup hugetlb file after mremap test")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Mina Almasry <almasrymina@google.com>
Reviewed-by: Muchun Song <songmuchun@bytedance.com>
Cc: Mike Kravetz <mike.kravetz@oracle.com>
Cc: Shuah Khan <shuah@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Author: Dan Carpenter

tools/testing/selftests/vm/hugepage-mremap.c

@@ -107,7 +107,7 @@ static void register_region_with_uffd(char *addr, size_t len)
 
 int main(int argc, char *argv[])
 {
-	size_t length;
+	size_t length = 0;
 
 	if (argc != 2 && argc != 3) {
 		printf("Usage: %s [length_in_MB] <hugetlb_file>\n", argv[0]);