net: Don't return invalid table id error when dumping all families

When doing a route dump across all address families, do not error out
if the table does not exist. This allows a route dump for AF_UNSPEC
with a table id that may only exist for some of the families.

Do return the table does not exist error if dumping routes for a
specific family and the table does not exist.

Signed-off-by: David Ahern <dsahern@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/ipv4/ipmr.c b/net/ipv4/ipmr.c
index 7a3e2ac..a6defbe 100644
--- a/net/ipv4/ipmr.c
+++ b/net/ipv4/ipmr.c
@@ -2542,6 +2542,9 @@ static int ipmr_rtm_dumproute(struct sk_buff *skb, struct netlink_callback *cb)
 
 		mrt = ipmr_get_table(sock_net(skb->sk), filter.table_id);
 		if (!mrt) {
+			if (filter.dump_all_families)
+				return skb->len;
+
 			NL_SET_ERR_MSG(cb->extack, "ipv4: MR table does not exist");
 			return -ENOENT;
 		}