When dev_set_promiscuity failed in macvlan_open, it always invokes
dev_set_allmulti without checking if necessary.
Now check the IFF_ALLMULTI flag firstly before rollback the multicast
setting in the error handler.

Signed-off-by: Gao Feng <fgao@ikuai8.com>
Signed-off-by: David S. Miller <davem@davemloft.net>