selftests: forwarding: mirror_lib: skip_hw the VLAN capture

When the VLAN capture is installed on a front panel device and not a
soft device, the packets are counted twice: once in fast path, and once
after they are trapped to the kernel. Resolve the problem by passing
skip_hw flag to vlan_capture_install().

Signed-off-by: Petr Machata <petrm@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>