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


git-svn-id: svn+ssh://svn.freebsd.org/base/head@272796 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f