mptcp: expose server_side attribute in MPTCP netlink events

This change records the server_side attribute in MPTCP_EVENT_CREATED
and MPTCP_EVENT_ESTABLISHED events to inform the recipient of the role
of the associated MPTCP application (Client/Server) that is handling
it's end of the MPTCP connection.

Closes: multipath-tcp/mptcp_net-next#246
Signed-off-by: Kishen Maloor <kishen.maloor@intel.com>

include/uapi/linux/mptcp.h

@@ -188,6 +188,7 @@ enum mptcp_event_attr {
 	MPTCP_ATTR_IF_IDX,	/* s32 */
 	MPTCP_ATTR_RESET_REASON,/* u32 */
 	MPTCP_ATTR_RESET_FLAGS, /* u32 */
+	MPTCP_ATTR_SERVER_SIDE,	/* u8 */
 
 	__MPTCP_ATTR_AFTER_LAST
 };

net/mptcp/pm_netlink.c

@@ -2097,6 +2097,9 @@ static int mptcp_event_created(struct sk_buff *skb,
 	if (err)
 		return err;
 
+	if (nla_put_u8(skb, MPTCP_ATTR_SERVER_SIDE, READ_ONCE(msk->pm.server_side)))
+		return -EMSGSIZE;
+
 	return mptcp_event_add_subflow(skb, ssk);
 }