iscsi-target: Fix memory corruption in iscsit_logout_post_handler_diffcid

This patch fixes a bug in iscsit_logout_post_handler_diffcid() where
a pointer used as storage for list_for_each_entry() was incorrectly
being used to determine if no matching entry had been found.

This patch changes iscsit_logout_post_handler_diffcid() to key off
bool conn_found to determine if the function needs to exit early.

Reported-by: Joern Engel <joern@logfs.org>
Cc: <stable@vger.kernel.org> # v3.1+
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>

drivers/target/iscsi/iscsi_target.c

@@ -4540,6 +4540,7 @@ static void iscsit_logout_post_handler_diffcid(
 {
 	struct iscsi_conn *l_conn;
 	struct iscsi_session *sess = conn->sess;
+	bool conn_found = false;
 
 	if (!sess)
 		return;
@@ -4548,12 +4549,13 @@ static void iscsit_logout_post_handler_diffcid(
 	list_for_each_entry(l_conn, &sess->sess_conn_list, conn_list) {
 		if (l_conn->cid == cid) {
 			iscsit_inc_conn_usage_count(l_conn);
+			conn_found = true;
 			break;
 		}
 	}
 	spin_unlock_bh(&sess->conn_lock);
 
-	if (!l_conn)
+	if (!conn_found)
 		return;
 
 	if (l_conn->sock)