Move the calls to u_tun_func() into udp6_append()

A similar cleanup for UDPv4 was performed in r220620.

Phabricator:	https://reviews.freebsd.org/D383
Reviewed by:	gnn
MFC after:	1 month
Use proper variable when looping through periphs with CAM_PERIPH_FREE.

PR:		194256
Submitted by:	Scott M. Ferris <smferris@gmail.com>
MFC after:	3 days
Sponsored by:	EMC/Isilon Storage Division