vswitch: Restore MAC learning for broadcast ARP replies on bonds.

vswitch: Restore MAC learning for broadcast ARP replies on bonds.

Bonding has a special exception for MAC learning: don't learn from packets
on bonded ports if we already have learned it on another port.  This is
because packets sent out one port can be received on the other, which would
cause us to learn incorrect locations.

But we need to make an exception for broadcast ARP replies, which indicate
that the MAC in question has moved to another switch.  Before commit
76fdb7e57 "Implement OFPP_NORMAL action in secchan and hook into vswitchd"
we did so, and this commit restores that behavior.