Ensure inp_vflag is consistently set for TCP endpoints.

Make sure that the flags INP_IPV4 and INP_IPV6 are consistently set
for inpcbs used for TCP sockets, no matter if the setting is derived
from the net.inet6.ip6.v6only sysctl or the IPV6_V6ONLY socket option.
For UDP this was already done right.

PR: 221385
MFC after: 1 week