MFC r339012:
For changing the MTU on tun/tap devices, it should not matter whether it
is done via using ifconfig, which uses a SIOCSIFMTU ioctl() command, or
doing it using a TUNSIFINFO/TAPSIFINFO ioctl() command.
Without this patch, for IPv6 the new MTU is not used when creating routes.
Especially, when initiating TCP connections after increasing the MTU,
the old MTU is still used to compute the MSS.
Thanks to ae@ and bz@ for helping to improve the patch.

Reviewed by: ae@, bz@
Sponsored by: Netflix, Inc.
Differential Revision: https://reviews.freebsd.org/D17180