bonding: 3ad: convert to bond->mode_lock

Now that we have bond->mode_lock, we can remove the state_machine_lock
and use it in its place. There're no fast paths requiring the per-port
spinlocks so it should be okay to consolidate them into mode_lock.
Also move it inside the unbinding function as we don't want to expose
mode_lock outside of the specific modes.

Suggested-by: Jay Vosburgh <jay.vosburgh@canonical.com>
Signed-off-by: Nikolay Aleksandrov <nikolay@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>