net: add missing includes and forward declarations under net/

This patch adds missing includes to headers under include/net.
All these problems are currently masked by the existing users
including the missing dependency before the broken header.

Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/inet_frag.h

@@ -4,6 +4,9 @@
 
 #include <linux/rhashtable-types.h>
 #include <linux/completion.h>
+#include <linux/in6.h>
+#include <linux/rbtree_types.h>
+#include <linux/refcount.h>
 
 /* Per netns frag queues directory */
 struct fqdir {