[tcp] Allow out-of-order receive queue to be discarded

Allow packets in the receive queue to be discarded in order to free up
memory.  This avoids a potential deadlock condition in which the
missing packet can never be received because the receive queue is
occupying all of the memory available for further RX buffers.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/include/ipxe/list.h

@@ -162,6 +162,18 @@ static inline int list_empty ( const struct list_head *head ) {
 	      &pos->member != (head);					      \
 	      pos = list_entry ( pos->member.next, typeof ( *pos ), member ) )
 
+/**
+ * Iterate over entries in a list in reverse order
+ *
+ * @v pos	The type * to use as a loop counter
+ * @v head	The head for your list
+ * @v member	The name of the list_struct within the struct
+ */
+#define list_for_each_entry_reverse( pos, head, member )		      \
+	for ( pos = list_entry ( (head)->prev, typeof ( *pos ), member );     \
+	      &pos->member != (head);					      \
+	      pos = list_entry ( pos->member.prev, typeof ( *pos ), member ) )
+
 /**
  * Iterate over entries in a list, safe against deletion of entries
  *