Don't drop DHCP packets on unsupported VLANs (BVS-2176)

Reviewer: @ducwindow @sdmodi
The DHCP agent runs for all packet-ins, regardless of whether DHCP offload was supposed to be enabled for a particular port and VLAN. If a DHCP packet was sent to the CPU for a non-DHCP reason (like an L3 route miss) we still want it to go to the controller. Basically the DHCP agent should only drop packets that it's very sure it's supposed to handle. In the future we should add a check on the in_port to see if DHCP offload was enabled for it.