bonding: 3ad: clean up curr_slave_lock usage

Remove the read_lock in bond_3ad_lacpdu_recv() since when the slave is
being released its rx_handler is removed before 3ad unbind, so even if
packets arrive, they won't see the slave in an inconsistent state.

Signed-off-by: Nikolay Aleksandrov <nikolay@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>