Implement 'q_swap' function

Use 'first' and 'second' pointers
to traverse the list and detect
if there is a pair of nodes.
Remove the 'first' node in the
pair using the 'list_del' API,
then swap the 'first' and 'second'
 nodes by inserting the 'first'
 node after the 'second' node
 using the  'list_add' API.

Change-Id: Id008f936abdb04455ec9ac4ba19f45759ff57757

Author: PoChuan994

queue.c

@@ -173,6 +173,16 @@ bool q_delete_dup(struct list_head *head)
 void q_swap(struct list_head *head)
 {
     // https://leetcode.com/problems/swap-nodes-in-pairs/
+    if (!head)
+        return;
+
+    struct list_head *first = head->next;
+    struct list_head *second = first->next;
+    for (; first != head && second != head;
+         first = first->next, second = first->next) {
+        list_del(first);
+        list_add(first, second);
+    }
 }
 
 /* Reverse elements in queue */