selftest/vm: uninitialized variable in main()

Initialize "length" to zero by default.

Link: https://lkml.kernel.org/r/YtZzjvHXVXMXxpXO@kili
Fixes: ff712a627f72 ("selftests/vm: cleanup hugetlb file after mremap test")
Signed-off-by: Dan Carpenter <[e-mail]>
Reviewed-by: Mina Almasry <[e-mail]>
Reviewed-by: Muchun Song <[e-mail]>
Cc: Mike Kravetz <[e-mail]>
Cc: Shuah Khan <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>

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

···
 
 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]);