bonding: fix length of actor system

The attribute IFLA_BOND_AD_ACTOR_SYSTEM is sent to user space having the
length of sizeof(bond->params.ad_actor_system) which is 8 byte. This
patch aligns the length to ETH_ALEN to have the same MAC address exposed
as using sysfs.

Fixes: f87fda00

 ("bonding: prevent out of bound accesses")
Signed-off-by: Tobias Jungel <tobias.jungel@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>