To overcome hardware checksum offload bug msk(4) used to compute

To overcome hardware checksum offload bug msk(4) used to compute
TCP/UDP checksum in driver for short frames. For frames that requires
hardware VLAN tag insertion, the checksum offload trick does not
work due to changes of checksum offset in mbuf after the VLAN tag.

Disable hardware checksum offload for VLAN interface to fix the bug.

Reported by:	Christopher Cowart < ccowart AT rescomp DOT berkeley DOT edu >
Tested by:	Christopher Cowart < ccowart AT rescomp DOT berkeley DOT edu >
MFC after:	5 days