Fix L2 caching for UDP over IPv6

ip6_output() was missing cache invalidation code analougous to
ip_output.c. r304545 disabled L2 caching for UDP/IPv6 as a workaround.
This change adds the missing cache invalidation code and reverts
r304545.

Reviewed by: gnn
Approved by: gnn (mentor)
Tested by: peter@, Mike Andrews
MFC after: 3 weeks
Differential Revision: https://reviews.freebsd.org/D7591