net: Correctly sync addresses from multiple sources to single device

When we have multiple devices attempting to sync the same address
to a single destination, each device should be permitted to sync
it once.  To accomplish this, pass the 'sync_cnt' of the source
address when adding the addresss to the lower device.  'sync_cnt'
tracks how many time a given address has been succefully synced.
This way, we know that if the 'sync_cnt' passed in is 0, we should
sync this address.

Also, turn 'synced' member back into the counter as was originally
done in
   commit 4543fbef.
   net: count hw_addr syncs so that unsync works properly.
It tracks how many time a given address has been added via a
'sync' operation.  For every successfull 'sync' the counter is
incremented, and for ever 'unsync', the counter is decremented.
This makes sure that the address will be properly removed from
the the lower device when all the upper devices have removed it.

Reported-by: Andrey Dmitrov <andrey.dmitrov@oktetlabs.ru>
CC: Andrey Dmitrov <andrey.dmitrov@oktetlabs.ru>
CC: Alexandra N. Kossovsky <Alexandra.Kossovsky@oktetlabs.ru>
CC: Konstantin Ushakov <Konstantin.Ushakov@oktetlabs.ru>
Signed-off-by: Vlad Yasevich <vyasevic@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>