net: count hw_addr syncs so that unsync works properly.

[ Upstream commit 4543fbefe6e06a9e40d9f2b28d688393a299f079 ]

A few drivers use dev_uc_sync/unsync to synchronize the
address lists from master down to slave/lower devices.  In
some cases (bond/team) a single address list is synched down
to multiple devices.  At the time of unsync, we have a leak
in these lower devices, because "synced" is treated as a
boolean and the address will not be unsynced for anything after
the first device/call.

Treat "synced" as a count (same as refcount) and allow all
unsync calls to work.

Signed-off-by: Vlad Yasevich <vyasevic@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>