s390/qeth: tighten ucast IP locking

The programming of ucast IPs via qeth_l3_modify_ip() is driven
independently from any of our typical locking mechanisms (eg. detaching
the netdevice, or holding the conf_mutex).
So when we inspect the card state to check whether the required cmd IO
should be deferred, there is no protection against concurrent state
changes.

But by slightly re-ordering the teardown sequence, we can rely on the
ip_lock to sufficiently serialize things:

1. when running concurrently to qeth_l3_set_online(), any instance of
   qeth_l3_modify_ip() that aquires the ip_lock _after_
   qeth_l3_recover_ip() will observe the state as CARD_STATE_SOFTSETUP
   and not defer the IO.
2. when running concurrently to qeth_l3_set_offline(), any instance of
   qeth_l3_modify_ip() that aquires the ip_lock _after_
   qeth_l3_clear_ip_htable() will observe the state as CARD_STATE_DOWN
   and defer the IO.

These guarantees in mind, we can now drop the conf_mutex from the
qeth_l3_modify_rxip_vipa() wrapper.

Signed-off-by: Julian Wiedmann <jwi@linux.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>