Factor out tcp6_use_min_mtu() to handle IPV6_USE_MIN_MTU by TCP.
Pass control for IP/IP6 level options from generic tcp_ctloutput_set()
down to per-stack ctloutput.
Call tcp6_use_min_mtu() from tcp stack tcp_default_ctloutput().
Reviewed by: rrs
Differential Revision: https://reviews.freebsd.org/D32655