Allow set ether/vlan PCP operation from the VNET jails.

The vlan interfaces can be created from vnet jails, it seems, so it
sounds logical to allow pcp configuration as well.

Reviewed by: bz, hselasky (previous version)
Sponsored by: Mellanox Technologies
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D17777