Set the inp_vflag consistently for accepted TCP/IPv6 connections when
net.inet6.ip6.v6only=0.

Without this patch, the inp_vflag would have INP_IPV4 and the
INP_IPV6 flags for accepted TCP/IPv6 connections if the sysctl
variable net.inet6.ip6.v6only is 0. This resulted in netstat
to report the source and destination addresses as IPv4 addresses,
even they are IPv6 addresses.

PR: 226421
Reviewed by: bz, hiren, kib
MFC after: 3 days
Sponsored by: Netflix, Inc.
Differential Revision: https://reviews.freebsd.org/D13514