MFC r304545: Disable L2 caching for UDP over IPv6

The ip6_output routine is missing L2 cache invalication as done
in ip_output. Even with that code, some problems with UDP over
IPv6 have been reported. Diabling L2 cache for that problem works
around the problem for now.

PR: 211872 211926
Reviewed by: gnn
Approved by: gnn (mentor)
Tested by: peter@, Mike Andrews
MFC after: immediate