[IPV6]: Remove circular dependency on if_inet6.h

net/if_inet6.h includes linux/ipv6.h which also tries to include
net/if_inet6.h.  Since the latter only needs it for forward
declarations, we can fix this by adding the declarations.

A number of files are implicitly including net/if_inet6.h through
linux/ipv6.h.  They also use net/ipv6.h so this patch includes
net/if_inet6.h there.
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/ipv6.h

@@ -219,7 +219,6 @@ enum {
 #include <linux/tcp.h>
 #include <linux/udp.h>
 
-#include <net/if_inet6.h>       /* struct ipv6_mc_socklist */
 #include <net/inet_sock.h>
 
 static inline struct ipv6hdr *ipv6_hdr(const struct sk_buff *skb)
@@ -273,6 +272,10 @@ struct tcp6_request_sock {
 	struct inet6_request_sock tcp6rsk_inet6;
 };
 
+struct ipv6_mc_socklist;
+struct ipv6_ac_socklist;
+struct ipv6_fl_socklist;
+
 /**
  * struct ipv6_pinfo - ipv6 private area
  *

include/net/ipv6.h

@@ -17,6 +17,7 @@
 
 #include <linux/ipv6.h>
 #include <linux/hardirq.h>
+#include <net/if_inet6.h>
 #include <net/ndisc.h>
 #include <net/flow.h>
 #include <net/snmp.h>