kernel: add accidentally left out fib trie backport patch

Signed-off-by: Felix Fietkau <nbd@openwrt.org>

SVN-Revision: 45650

target/linux/generic/patches-3.18/080-19-fib_trie-Use-index-0ul-n-bits-instead-of-index-n-bit.patch

@@ -0,0 +1,52 @@
+From: Alexander Duyck <alexander.h.duyck@redhat.com>
+Date: Thu, 22 Jan 2015 15:51:08 -0800
+Subject: [PATCH] fib_trie: Use index & (~0ul << n->bits) instead of index >>
+ n->bits
+
+In doing performance testing and analysis of the changes I recently found
+that by shifting the index I had created an unnecessary dependency.
+
+I have updated the code so that we instead shift a mask by bits and then
+just test against that as that should save us about 2 CPU cycles since we
+can generate the mask while the key and pos are being processed.
+
+Signed-off-by: Alexander Duyck <alexander.h.duyck@redhat.com>
+Signed-off-by: David S. Miller <davem@davemloft.net>
+---
+
+--- a/net/ipv4/fib_trie.c
++++ b/net/ipv4/fib_trie.c
+@@ -961,12 +961,12 @@ static struct tnode *fib_find_node(struc
+ 		 * prefix plus zeros for the bits in the cindex. The index
+ 		 * is the difference between the key and this value.  From
+ 		 * this we can actually derive several pieces of data.
+-		 *   if !(index >> bits)
+-		 *     we know the value is cindex
+-		 *   else
++		 *   if (index & (~0ul << bits))
+ 		 *     we have a mismatch in skip bits and failed
++		 *   else
++		 *     we know the value is cindex
+ 		 */
+-		if (index >> n->bits)
++		if (index & (~0ul << n->bits))
+ 			return NULL;
+ 
+ 		/* we have found a leaf. Prefixes have already been compared */
+@@ -1301,12 +1301,12 @@ int fib_table_lookup(struct fib_table *t
+ 		 * prefix plus zeros for the "bits" in the prefix. The index
+ 		 * is the difference between the key and this value.  From
+ 		 * this we can actually derive several pieces of data.
+-		 *   if !(index >> bits)
+-		 *     we know the value is child index
+-		 *   else
++		 *   if (index & (~0ul << bits))
+ 		 *     we have a mismatch in skip bits and failed
++		 *   else
++		 *     we know the value is cindex
+ 		 */
+-		if (index >> n->bits)
++		if (index & (~0ul << n->bits))
+ 			break;
+ 
+ 		/* we have found a leaf. Prefixes have already been compared */