bonding: remove unnecessary dev_addr_from_first member

In struct bonding there's a member called dev_addr_from_first which is
used to denote when the bond dev should clone the first slave's MAC
address but since we have netdev's addr_assign_type variable that is not
necessary. We clone the first slave's MAC each time we have a random MAC
set to the bond device. This has the nice side-effect of also fixing an
inconsistency - when the MAC address of the bond dev is set after its
creation, but prior to having slaves, it's not kept and the first slave's
MAC is cloned. The only way to keep the MAC was to create the bond device
with the MAC address set (e.g. through ip link). In all cases if the
bond device is left without any slaves - its MAC gets reset to a random
one as before.

Signed-off-by: Nikolay Aleksandrov <nikolay@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>