selftest/vm: uninitialized variable in main()

Initialize "length" to zero by default.

Link: https://lkml.kernel.org/r/YtZzjvHXVXMXxpXO@kili
Fixes: ff712a62

 ("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>