mptcp: pm: netlink: deprecate server-side attribute

Now that such info is in the 'flags' attribute, it is time to deprecate
the dedicated 'server-side' attribute.

It will be 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]>

Documentation/netlink/specs/mptcp_pm.yaml

···
       -
         name: server-side
         type: u8
+        doc: "Deprecated: use 'flags'"
 
 operations:
   list:

net/mptcp/pm_netlink.c

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