mptcp: Add MPTCP to skb extensions

Add enum value for MPTCP and update config dependencies

v5 -> v6:
 - fixed '__unused' field size

Co-developed-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Co-developed-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/MAINTAINERS b/MAINTAINERS
index 218089f..9dcb9ca 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -11573,6 +11573,16 @@
 F:	net/netfilter/xt_CONNSECMARK.c
 F:	net/netfilter/xt_SECMARK.c
 
+NETWORKING [MPTCP]
+M:	Mat Martineau <mathew.j.martineau@linux.intel.com>
+M:	Matthieu Baerts <matthieu.baerts@tessares.net>
+L:	netdev@vger.kernel.org
+L:	mptcp@lists.01.org
+W:	https://github.com/multipath-tcp/mptcp_net-next/wiki
+B:	https://github.com/multipath-tcp/mptcp_net-next/issues
+S:	Maintained
+F:	include/net/mptcp.h
+
 NETWORKING [TCP]
 M:	Eric Dumazet <edumazet@google.com>
 L:	netdev@vger.kernel.org