epair: Avoid loading m_flags into a short

The m_flags field of struct mbuf is 24 bits wide and so gets truncated
in a couple of places in the epair code. Instead of preserving the
entire flag set, just remember whether M_BCAST or M_MCAST is set.

MFC after: 1 week
Sponsored by: Klara, Inc.

(cherry picked from commit 48227d1c6db8fceaceebbf8578612302d64ca170)