selftests/bpf: Fix vmtest.sh to launch smp vm.

Fix typo in vmtest.sh to make sure it launch proper vm with 8 cpus.

Signed-off-by: Yucong Sun <fallentree@fb.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20220217155212.2309672-1-fallentree@fb.com