[NET]: Messed multicast lists after dev_mc_sync/unsync

Commit a0a400d79e3dd7843e7e81baa3ef2957bdc292d0 ("[NET]: dev_mcast:
add multicast list synchronization helpers") from you introduced a new
field "da_synced" to struct dev_addr_list that is not properly
initialized to 0. So when any of the current users (8021q, macvlan,
mac80211) calls dev_mc_sync/unsync they mess the address list for both
devices.

The attached patch fixed it for me and avoid future problems.

Signed-off-by: Jorge Boncompte [DTI2] <jorge@dti2.net>
Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>