vfs: don't open real

Let overlayfs do its thing when opening a file.

This enables stacking and fixes the corner case when a file is opened for
read, modified through a writable open, and data is read from the read-only
file.  After this patch the read-only open will not return stale data even
in this case.
Signed-off-by: NMiklos Szeredi <mszeredi@redhat.com>
Signed-off-by: NAl Viro <viro@zeniv.linux.org.uk>

fs/open.c

@@ -893,13 +893,8 @@ EXPORT_SYMBOL(file_path);
  */
 int vfs_open(const struct path *path, struct file *file)
 {
-	struct dentry *dentry = d_real(path->dentry, NULL, file->f_flags, 0);
-
-	if (IS_ERR(dentry))
-		return PTR_ERR(dentry);
-
 	file->f_path = *path;
-	return do_dentry_open(file, d_backing_inode(dentry), NULL);
+	return do_dentry_open(file, d_backing_inode(path->dentry), NULL);
 }
 
 struct file *dentry_open(const struct path *path, int flags,