ovl: fix relatime for directories

Need to treat non-regular overlayfs files the same as regular files when
checking for an atime update.

Add a d_real() flag to make it return the upper dentry for all file types.

Reported-by: "zhangyi (F)" <yi.zhang@huawei.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
diff --git a/include/linux/dcache.h b/include/linux/dcache.h
index fd0721e..ed1a7cf 100644
--- a/include/linux/dcache.h
+++ b/include/linux/dcache.h
@@ -562,6 +562,9 @@ static inline struct dentry *d_backing_dentry(struct dentry *upper)
 	return upper;
 }
 
+/* d_real() flags */
+#define D_REAL_UPPER	0x2	/* return upper dentry or NULL if non-upper */
+
 /**
  * d_real - Return the real dentry
  * @dentry: the dentry to query