mctp: Add MCTP base

Add basic Kconfig, an initial (empty) af_mctp source object, and
{AF,PF}_MCTP definitions, and the required definitions for a new
protocol type.

Signed-off-by: Jeremy Kerr <jk@codeconstruct.com.au>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/socket.c b/net/socket.c
index 42665bd..3c10504 100644
--- a/net/socket.c
+++ b/net/socket.c
@@ -212,6 +212,7 @@ static const char * const pf_family_names[] = {
 	[PF_QIPCRTR]	= "PF_QIPCRTR",
 	[PF_SMC]	= "PF_SMC",
 	[PF_XDP]	= "PF_XDP",
+	[PF_MCTP]	= "PF_MCTP",
 };
 
 /*