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
MFV r272802:

 - Limit ARC for zdb at 256MB.  zdb do not typically revisit data
   in the ARC.
 - Increase default max_inflight from 200 to 1000 (can be overriden
   by -I) so we can queue more I/Os when doing scrubbing.
 - Print status while loading meataslabs for leak detection.

Illumos issues:

    5169 zdb should limit its ARC size
    5170 zdb -c should create more scrub i/os by default
    5171 zdb should print status while loading metaslabs for leak detection

MFC after:	2 weeks