[SCSI] be2iscsi: fix lock imbalance

Stanse found that one error path in mgmt_invalidate_icds omits to unlock
ctrl->mbox_lock. Fix that.

Added in 756d29c8 (Enable async mode for mcc rings)
where the spinlock was moved.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Acked-by: Jayamohan Kallickal <jayamohank@serverengines.com>
Reviewed-by: Mike Christie <michaelc@cs.wisc.edu>
Signed-off-by: James Bottomley <James.Bottomley@suse.de>