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

tools/testing/selftests/bpf/vmtest.sh

@@ -241,7 +241,7 @@ EOF
 		-nodefaults \
 		-display none \
 		-serial mon:stdio \
-		"${qemu_flags[@]}" \
+		"${QEMU_FLAGS[@]}" \
 		-enable-kvm \
 		-m 4G \
 		-drive file="${rootfs_img}",format=raw,index=1,media=disk,if=virtio,cache=none \