Add M_FLOWID to M_COPYFLAGS

The M_FLOWID flag should be propagated to the new mbuf pkthdr in
m_move_pkthdr() and m_dup_pkthdr(). The new mbuf already got the
existing flowid value, but would be ignored since the flag was
not set.

Phabricator: https://reviews.freebsd.org/D914
Reviewed by: adrian
Obtained from: NetApp
MFC after: 1 week