[DECNET]: add memory buffer settings

The patch (originally from Steve) simply adds memory buffer settings to 
DECnet similar to those in TCP.

Signed-off-by: Patrick Caulfield <patrick@tykepenguin.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/sysctl.h

@@ -670,6 +670,9 @@ enum {
 	NET_DECNET_DST_GC_INTERVAL = 9,
 	NET_DECNET_CONF = 10,
 	NET_DECNET_NO_FC_MAX_CWND = 11,
+	NET_DECNET_MEM = 12,
+	NET_DECNET_RMEM = 13,
+	NET_DECNET_WMEM = 14,
 	NET_DECNET_DEBUG_LEVEL = 255
 };