selftests/bpf: Correct the INDEX address in vmtest.sh

Migration of vmtest to libbpf/ci will change the address
of INDEX in vmtest.sh, which will cause vmtest.sh to not
work due to the failure of rootfs fetching.

Signed-off-by: Pu Lehui <pulehui@huawei.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Tested-by: Lorenzo Fontana <lorenzo.fontana@elastic.co>
Link: https://lore.kernel.org/bpf/20211220050803.2670677-1-pulehui@huawei.com