datapath: Fix VLAN tag insertion actions on Xen.

datapath: Fix VLAN tag insertion actions on Xen.

On Xen, a VM can pass a packet that needs to be checksummed up to Dom0 for
transmission on the wire.  In this case, Dom0 is supposed to pick apart
the packet, figure out the protocol, and checksum it before sending it out.
However, this fails if we insert an 802.1Q header, because the Xen routine
that picks apart packets (skb_checksum_setup() in net/core/dev.c) does not
understand 802.1Q.  Hence, we must call this function ourselves, before we
add the 802.1Q header.

Fixes bug #1215.