[Bug 176671] [epair] MAC address for epair device not unique

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=176671

--- Comment #6 from Olivier Cochard <olivier at freebsd.org> ---
I've reach to reproduce a setup when this localy-unique epair create conflict
on a LAN:

     +----------------+       +---------------+
     |     ServerA    |       |    serverB    |
     |  bridge0 - em0 |- LAN -| em0 - bridge0 |
     |    |           |       |         |     |
     |  epair1a       |       |       epair1a |
     |  epair1b       |       |       epair1b |
     |    |           |       |         |     |
     |  vnetjail1     |       |     vnetjail2 |
     +----------------+       +---------------+

MAC addresses of epair1 are the same on both jail, then they can't communicate
each others:

[root at jail1]~# ifconfig epair1b ether
epair1b: flags=8943<UP,BROADCAST,RUNNING,PROMISC,SIMPLEX,MULTICAST> metric 0
mtu 1500
        options=8<VLAN_MTU>
        ether 02:ff:70:00:0f:0b
        hwaddr 02:ff:70:00:0f:0b


[root at jail2]~# ifconfig epair1b ether
epair1b: flags=8943<UP,BROADCAST,RUNNING,PROMISC,SIMPLEX,MULTICAST> metric 0
mtu 1500
        options=8<VLAN_MTU>
        ether 02:ff:70:00:0f:0b
        hwaddr 02:ff:70:00:0f:0b

We could perhaps generate a MAC field "eaddr[1]" (in if_epair.c) from the
hostid  for mitigate this conflict (like the net/if_bridge.c code) ?
But if we take code from if_bridge, we should create a common function for
being available to if_bridge and if_epair.

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