[INET]: Introduce tunnel4/tunnel6

Basically this patch moves the generic tunnel protocol stuff out of
xfrm4_tunnel/xfrm6_tunnel and moves it into the new files of tunnel4.c
and tunnel6 respectively.

The reason for this is that the problem that Hugo uncovered is only
the tip of the iceberg.  The real problem is that when we removed the
dependency of ipip on xfrm4_tunnel we didn't really consider the module
case at all.

For instance, as it is it's possible to build both ipip and xfrm4_tunnel
as modules and if the latter is loaded then ipip simply won't load.

After considering the alternatives I've decided that the best way out of
this is to restore the dependency of ipip on the non-xfrm-specific part
of xfrm4_tunnel.  This is acceptable IMHO because the intention of the
removal was really to be able to use ipip without the xfrm subsystem.
This is still preserved by this patch.

So now both ipip/xfrm4_tunnel depend on the new tunnel4.c which handles
the arbitration between the two.  The order of processing is determined
by a simple integer which ensures that ipip gets processed before
xfrm4_tunnel.

The situation for ICMP handling is a little bit more complicated since
we may not have enough information to determine who it's for.  It's not
a big deal at the moment since the xfrm ICMP handlers are basically
no-ops.  In future we can deal with this when we look at ICMP caching
in general.

The user-visible change to this is the removal of the TUNNEL Kconfig
prompts.  This makes sense because it can only be used through IPCOMP
as it stands.

The addition of the new modules shouldn't introduce any problems since
module dependency will cause them to be loaded.

Oh and I also turned some unnecessary pskb's in IPv6 related to this
patch to skb's.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv6/Kconfig

@@ -88,7 +88,7 @@ config INET6_IPCOMP
 	tristate "IPv6: IPComp transformation"
 	depends on IPV6
 	select XFRM
-	select INET6_TUNNEL
+	select INET6_XFRM_TUNNEL
 	select CRYPTO
 	select CRYPTO_DEFLATE
 	---help---
@@ -97,19 +97,18 @@ config INET6_IPCOMP
 
 	  If unsure, say Y.
 
+config INET6_XFRM_TUNNEL
+	tristate
+	select INET6_TUNNEL
+	default n
+
 config INET6_TUNNEL
-	tristate "IPv6: tunnel transformation"
-	depends on IPV6
-	select XFRM
-	---help---
-	  Support for generic IPv6-in-IPv6 tunnel transformation, which is
-	  required by the IPv6-in-IPv6 tunneling module as well as tunnel mode
-	  IPComp.
-	  
-	  If unsure, say Y.
+	tristate
+	default n
 
 config IPV6_TUNNEL
 	tristate "IPv6: IPv6-in-IPv6 tunnel"
+	select INET6_TUNNEL
 	depends on IPV6
 	---help---
 	  Support for IPv6-in-IPv6 tunnels described in RFC 2473.