scsi: libiscsi: Annotate locking assumptions

This patch avoids that sparse reports the following:

drivers/scsi/libiscsi.c:1844:23: warning: context imbalance in 'iscsi_exec_task_mgmt_fn' - unexpected unlock

Signed-off-by: Bart Van Assche <bart.vanassche@wdc.com>
Signed-off-by: Lee Duncan <lduncan@suse.com>
Signed-off-by: Chris Leech <cleech@redhat.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>