ipv6: Use __fls() instead of fls() in __ipv6_addr_diff().

Because we have ensured that the argument is non-zero,
it is better to use __fls() and generate better code.
Signed-off-by: YOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/ipv6.h

@@ -422,7 +422,7 @@ static inline int __ipv6_addr_diff(const void *token1, const void *token2, int a
 	for (i = 0; i < addrlen; i++) {
 		__be32 xb = a1[i] ^ a2[i];
 		if (xb)
-			return i * 32 + 32 - fls(ntohl(xb));
+			return i * 32 + 31 - __fls(ntohl(xb));
 	}
 
 	/*