net: rtnetlink: rtnl_stats_get(): Emit an extack for unset filter_mask

Both get and dump handlers for RTM_GETSTATS require that a filter_mask, a
mask of which attributes should be emitted in the netlink response, is
unset. rtnl_stats_dump() does include an extack in the bounce,
rtnl_stats_get() however does not. Fix the omission.
Signed-off-by: Petr Machata <petrm@nvidia.com>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Link: https://lore.kernel.org/r/01feb1f4bbd22a19f6629503c4f366aed6424567.1645020876.git.petrm@nvidia.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>

net/core/rtnetlink.c

@@ -5405,8 +5405,10 @@ static int rtnl_stats_get(struct sk_buff *skb, struct nlmsghdr *nlh,
 		return -ENODEV;
 
 	filter_mask = ifsm->filter_mask;
-	if (!filter_mask)
+	if (!filter_mask) {
+		NL_SET_ERR_MSG(extack, "Filter mask must be set for stats get");
 		return -EINVAL;
+	}
 
 	nskb = nlmsg_new(if_nlmsg_stats_size(dev, filter_mask), GFP_KERNEL);
 	if (!nskb)