fs: fix Inode.IsDir logic

The file type is not a bitmap, and looking at

	S_IFBLK                          = 0x6000
	S_IFCHR                          = 0x2000
	S_IFDIR                          = 0x4000
	S_IFIFO                          = 0x1000
	S_IFLNK                          = 0xa000
	S_IFREG                          = 0x8000
	S_IFSOCK                         = 0xc000

and confirmed by the added test, we used to classify
block devices and sockets as directories:

 --- FAIL: TestInodeIsDir (0.00s)
     inode_test.go:25: wrong result for case struct { mode uint32; dir bool }{mode:0x6000, dir:false}
     inode_test.go:25: wrong result for case struct { mode uint32; dir bool }{mode:0xc000, dir:false}

The check is fixed now and the test passes.

Change-Id: I6490992d1fecc8a6bea7c2c4b2f1bca765d03d4c

fs/inode.go

@@ -104,7 +104,7 @@ type Inode struct {
 }
 
 func (n *Inode) IsDir() bool {
-	return n.stableAttr.Mode&syscall.S_IFDIR != 0
+	return n.stableAttr.Mode&syscall.S_IFMT == syscall.S_IFDIR
 }
 
 func (n *Inode) embed() *Inode {

fs/inode_test.go

+package fs
+
+import (
+	"syscall"
+	"testing"
+)
+
+func TestInodeIsDir(t *testing.T) {
+	cases := []struct {
+		mode uint32
+		dir  bool
+	}{
+		{syscall.S_IFBLK, false},
+		{syscall.S_IFCHR, false},
+		{syscall.S_IFDIR, true},
+		{syscall.S_IFIFO, false},
+		{syscall.S_IFLNK, false},
+		{syscall.S_IFREG, false},
+		{syscall.S_IFSOCK, false},
+	}
+	var i Inode
+	for _, c := range cases {
+		i.stableAttr.Mode = c.mode
+		if i.IsDir() != c.dir {
+			t.Errorf("wrong result for case %#v", c)
+		}
+	}
+}