MINOR: mt_lists: Do nothing in MT_LIST_ADD/MT_LIST_ADDQ if already in list.

Modify MT_LIST_ADD and MT_LIST_ADDQ to do nothing if the element is already
in a list.
diff --git a/include/common/mini-clist.h b/include/common/mini-clist.h
index 642a7a3..3d51919 100644
--- a/include/common/mini-clist.h
+++ b/include/common/mini-clist.h
@@ -221,6 +221,11 @@
 				__ha_barrier_store();                      \
 				continue;                                  \
 			}                                                  \
+			if ((el)->next != (el) || (el)->prev != (el)) {    \
+				(n)->prev = p;                             \
+				(lh)->next = n;                            \
+				break;                                     \
+			}                                                  \
 			(el)->next = n;                                    \
 			(el)->prev = p;                                    \
 			__ha_barrier_store();                              \
@@ -246,6 +251,11 @@
 				__ha_barrier_store();                      \
 				continue;                                  \
 			}                                                  \
+			if ((el)->next != (el) || (el)->prev != (el)) {    \
+				p->next = n;                               \
+				(lh)->prev = p;                            \
+				break;                                     \
+			}                                                  \
 			(el)->next = n;                                    \
 			(el)->prev = p;                                    \
 			__ha_barrier_store();                              \