ovl: Do not expose metacopy only dentry from d_real()

Metacopy dentry/inode is internal to overlay and is never exposed outside
of it.  Exception is metacopy upper file used for fsync().  Modify d_real()
to look for dentries/inode which have data, but also allow matching upper
inode without data for the fsync case.
Signed-off-by: Vivek Goyal <vgoyal@redhat.com>
Reviewed-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>

fs/overlayfs/super.c

@@ -95,10 +95,13 @@ static struct dentry *ovl_d_real(struct dentry *dentry,
 	}
 
 	real = ovl_dentry_upper(dentry);
-	if (real && (!inode || inode == d_inode(real)))
+	if (real && (inode == d_inode(real)))
 		return real;
 
-	real = ovl_dentry_lower(dentry);
+	if (real && !inode && ovl_has_upperdata(d_inode(dentry)))
+		return real;
+
+	real = ovl_dentry_lowerdata(dentry);
 	if (!real)
 		goto bug;