scsi: libiscsi: Fix locking in __iscsi_conn_send_pdu

The code at free_task label in __iscsi_conn_send_pdu can get executed
from blk_timeout_work which takes queue_lock using spin_lock_irq.
back_lock taken with spin_unlock_bh will cause WARN_ON_ONCE.  The code
gets executed either with bottom half or IRQ disabled hence using
spin_lock/spin_unlock for back_lock is safe.
Signed-off-by: NJitendra Bhivare <jitendra.bhivare@broadcom.com>
Reviewed-by: NHannes Reinecke <hare@suse.com>
Reviewed-by: NChris Leech <cleech@redhat.com>
Signed-off-by: NMartin K. Petersen <martin.petersen@oracle.com>

drivers/scsi/libiscsi.c

@@ -791,9 +791,9 @@ __iscsi_conn_send_pdu(struct iscsi_conn *conn, struct iscsi_hdr *hdr,
 
 free_task:
 	/* regular RX path uses back_lock */
-	spin_lock_bh(&session->back_lock);
+	spin_lock(&session->back_lock);
 	__iscsi_put_task(task);
-	spin_unlock_bh(&session->back_lock);
+	spin_unlock(&session->back_lock);
 	return NULL;
 }