mptcp: pm: netlink: announce server-side flag

Now that the 'flags' attribute is used, it seems interesting to add one
flag for 'server-side', a boolean value.

This is duplicating the info from the dedicated 'server-side' attribute,
but it will be deprecated in the next commit, and removed in a few
versions.

Reviewed-by: Geliang Tang <[e-mail]>
Signed-off-by: Matthieu Baerts (NGI0) <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

include/uapi/linux/mptcp.h

···
 #define MPTCP_INFO_FLAG_REMOTE_KEY_RECEIVED	_BITUL(1)
 
 #define MPTCP_PM_EV_FLAG_DENY_JOIN_ID0		_BITUL(0)
+#define MPTCP_PM_EV_FLAG_SERVER_SIDE		_BITUL(1)
 
 #define MPTCP_PM_ADDR_FLAG_SIGNAL                      (1 << 0)
 #define MPTCP_PM_ADDR_FLAG_SUBFLOW                     (1 << 1)

net/mptcp/pm_netlink.c

···
 	if (err)
 		return err;
 
-	/* only set when it is the server side */
-	if (READ_ONCE(msk->pm.server_side) &&
-	    nla_put_u8(skb, MPTCP_ATTR_SERVER_SIDE, 1))
-		return -EMSGSIZE;
+	if (READ_ONCE(msk->pm.server_side)) {
+		flags |= MPTCP_PM_EV_FLAG_SERVER_SIDE;
+
+		/* only set when it is the server side */
+		if (nla_put_u8(skb, MPTCP_ATTR_SERVER_SIDE, 1))
+			return -EMSGSIZE;
+	}
 
 	if (READ_ONCE(msk->pm.remote_deny_join_id0))
 		flags |= MPTCP_PM_EV_FLAG_DENY_JOIN_ID0;