Add single-element cache for DirIV lookup

Another 3x performance boost for applications that walk the
directory tree.

Excerpt from performance.txt:

VERSION         UNTAR    LS     RM
v0.4               48     1.5    5
v0.5-rc1           56     7     19
v0.5-rc1-1         54     4.1    9
v0.5-rc1-2         45     1.7	 3.4  <---- THIS VERSION

1 files changed, 3 insertions, 3 deletions

diff --git a/cryptfs/kdf.go b/cryptfs/kdf.go
index 73be0bb..db08500 100644
--- a/cryptfs/kdf.go
+++ b/cryptfs/kdf.go
@@ -1,10 +1,10 @@
 package cryptfs
 
 import (
-	"os"
-	"math"
 	"fmt"
 	"golang.org/x/crypto/scrypt"
+	"math"
+	"os"
 )
 
 const (
@@ -50,5 +50,5 @@ func (s *scryptKdf) DeriveKey(pw string) []byte {
 // LogN - N is saved as 2^LogN, but LogN is much easier to work with.
 // This function gives you LogN = Log2(N).
 func (s *scryptKdf) LogN() int {
-	return int(math.Log2(float64(s.N))+0.5)
+	return int(math.Log2(float64(s.N)) + 0.5)
 }