os: make MkdirAll work with symlinks

Fixes #1149.

R=adg
CC=golang-dev
https://golang.org/cl/3564041
diff --git a/src/pkg/os/path.go b/src/pkg/os/path.go
index 74c83ab..b762971 100644
--- a/src/pkg/os/path.go
+++ b/src/pkg/os/path.go
@@ -14,7 +14,7 @@
 // and returns nil.
 func MkdirAll(path string, perm uint32) Error {
 	// If path exists, stop with success or error.
-	dir, err := Lstat(path)
+	dir, err := Stat(path)
 	if err == nil {
 		if dir.IsDirectory() {
 			return nil