[SCSI] iscsi class: fix lock dep warning on logout

We never should hit the lock up that is spit out when
lock dep is on and we logout. But we have been using the
shost work queue in a odd way. This patch has us use the
work queue for scanning instead of creating our own,
and this ends up also killing the lock dep warnings.

Signed-off-by: Mike Christie <michaelc@cs.wisc.edu>
Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>