neighbour: Remove NEIGH_DN_TABLE.

Since commit 1202cdd ("Remove DECnet support from kernel"),
NEIGH_DN_TABLE is no longer used.

MPLS has implicit dependency on it in nla_put_via(), but nla_get_via()
does not support DECnet.

Let's remove NEIGH_DN_TABLE.

Now, neigh_tables[] has only 2 elements and no extra iteration
for DECnet in many places.

Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Link: https://patch.msgid.link/20241014235216.10785-1-kuniyu@amazon.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

include/net/neighbour.h

@@ -239,7 +239,6 @@ struct neigh_table {
 enum {
 	NEIGH_ARP_TABLE = 0,
 	NEIGH_ND_TABLE = 1,
-	NEIGH_DN_TABLE = 2,
 	NEIGH_NR_TABLES,
 	NEIGH_LINK_TABLE = NEIGH_NR_TABLES /* Pseudo table for neigh_xmit */
 };

net/mpls/af_mpls.c

@@ -1664,7 +1664,7 @@ static int nla_put_via(struct sk_buff *skb,
 		       u8 table, const void *addr, int alen)
 {
 	static const int table_to_family[NEIGH_NR_TABLES + 1] = {
-		AF_INET, AF_INET6, AF_DECnet, AF_PACKET,
+		AF_INET, AF_INET6, AF_PACKET,
 	};
 	struct nlattr *nla;
 	struct rtvia *via;