ethersubr: Make the mac address generation more robust

If we create two (vnet) jails and create a bridge interface in each we end up
with the same mac address on both bridge interfaces.
These very often conflicts, resulting in same mac address in both jails.

Mitigate this problem by including the jail name in the mac address.

Reviewed by: kevans, melifaro
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D24383