rc: network.subr update consitency with older change (v6/v4 order)

As of 1b5be7204eaeeaf58eefdebe5b308f90792c693b we setup parts of IPv6
before IPv4 if configured. For consistency change a case in ifn_start()
calling ipv6_up() before ipv4_up() and reverse in ifn_stop().

MFC after: 10 days
Reviewed by: zlei
Differential Revision: https://reviews.freebsd.org/D33426