ctcm: suspend has to wait for outstanding I/O

State transition to DEV_STATE_STOPPED indicates all outstanding I/O has
finished. Add wait queue to wait for this state.

Signed-off-by: Frank Blaschka <frank.blaschka@de.ibm.com>
Signed-off-by: Ursula Braun <ursula.braun@de.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>