FIB: correctly report IPv6 FIB Unicast and Multicast memory usage (VPP-1578)"

and document scaling

Change-Id: I65d8999e65616d77e525963c770d91e9b0d5e593
Signed-off-by: Neale Ranns <nranns@cisco.com>
diff --git a/src/vnet/mfib/ip4_mfib.c b/src/vnet/mfib/ip4_mfib.c
index 8760371..1d872f9 100644
--- a/src/vnet/mfib/ip4_mfib.c
+++ b/src/vnet/mfib/ip4_mfib.c
@@ -366,7 +366,7 @@
         total_memory += mfib_size;
     }));
 
-    s = format(s, "%=30s %=6d %=8ld\n",
+    s = format(s, "%=30s %=6d %=12ld\n",
                "IPv4 multicast",
                pool_elts(ip4_main.mfibs), total_memory);