[L2TP]: Changes to existing ppp and socket kernel headers for L2TP

Add struct sockaddr_pppol2tp to carry L2TP-specific address
information for the PPPoX (PPPoL2TP) socket. Unfortunately we can't
use the union inside struct sockaddr_pppox because the L2TP-specific
data is larger than the current size of the union and we must preserve
the size of struct sockaddr_pppox for binary compatibility.

Also add a PPPIOCGL2TPSTATS ioctl to allow userspace to obtain
L2TP counters and state from the kernel.

Add new if_pppol2tp.h header.

[ Modified to use aligned_u64 in statistics structure -DaveM ]

Signed-off-by: James Chapman <jchapman@katalix.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/include/linux/socket.h b/include/linux/socket.h
index 6e7c948..fe195c9 100644
--- a/include/linux/socket.h
+++ b/include/linux/socket.h
@@ -287,6 +287,7 @@
 #define SOL_NETLINK	270
 #define SOL_TIPC	271
 #define SOL_RXRPC	272
+#define SOL_PPPOL2TP	273
 
 /* IPX options */
 #define IPX_TYPE	1