MFC r272796:

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.
Merge remote-tracking branch 'freebsd/stable/10' into hardened/10/aslr