selftests/vm/write_to_hugetlbfs.c: fix unused variable warning

Remove unused variable "i", which was triggering a compiler warning.

Fixes: 29750f71

 ("hugetlb_cgroup: add hugetlb_cgroup reservation tests")
Signed-off-by: John Hubbard <jhubbard@nvidia.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Reviewed-By: Mina Almasry <almasrymina@google.com>
Cc: Brian Geffon <bgeffon@google.com>
Cc: "Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>
Cc: Shuah Khan <shuah@kernel.org>
Cc: Ralph Campbell <rcampbell@nvidia.com>
Link: http://lkml.kernel.org/r/20200517001245.361762-2-jhubbard@nvidia.com
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>