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
Differential Revision:  https://reviews.freebsd.org/D7591