bonding: fix vlan_features computing

bond_compute_features() uses netdev_increment_features() to
combine vlan_features of slaves into vlan_features of the bond.
As netdev_increment_features() only adds most features and we
start with BOND_VLAN_FEATURES, we can end up with features none
of the slaves provided.

If there is at least one slave, initialize vlan_features only
with the flags in NETIF_F_ALL_FOR_ALL. Right now there is none
in BOND_VLAN_FEATURES but stating it explicitely will make the
code more future proof.

Signed-off-by: Michal Kubecek <mkubecek@suse.cz>
Signed-off-by: David S. Miller <davem@davemloft.net>