net/bonding: Export bond_option_active_slave_get_rcu

Some consumers of the netdev events API would like to know who is the
active slave when a NETDEV_CHANGEUPPER or NETDEV_BONDING_FAILOVER
events occur. For example, when managing RoCE GIDs, GIDs based on the
bond's ips should only be set on the port which corresponds to active
slave netdevice.

Signed-off-by: Matan Barak <matanb@mellanox.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>