IP6-MFIB: replace the radix tree with bihash (VPP-1526)

Change-Id: I7a48890c075826fbd8c75436dfdc5ffff230a693
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 eaa61c0..9d70f0b 100644
--- a/src/vnet/mfib/ip4_mfib.c
+++ b/src/vnet/mfib/ip4_mfib.c
@@ -210,7 +210,7 @@
         }
     }
 
-    for (mask_len = 32; mask_len >= 0; mask_len--)
+    for (mask_len = (len == 64 ? 32 : len); mask_len >= 0; mask_len--)
     {
         hash = mfib->fib_entry_by_dst_address[mask_len];
         IP4_MFIB_MK_GRP_KEY(grp, mask_len, key);