ovl: Add an inode flag OVL_CONST_INO

Add an ovl_inode flag OVL_CONST_INO.  This flag signifies if inode number
will remain constant over copy up or not.  This flag does not get updated
over copy up and remains unmodifed after setting once.

Next patch in the series will make use of this flag.  It will basically
figure out if dentry is of type ORIGIN or not.  And this can be derived by
this flag.

ORIGIN = (upperdentry && ovl_test_flag(OVL_CONST_INO, inode)).

Suggested-by: Amir Goldstein <amir73il@gmail.com>
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/inode.c

@@ -883,6 +883,9 @@ struct inode *ovl_get_inode(struct super_block *sb,
 
 	OVL_I(inode)->redirect = oip->redirect;
 
+	if (bylower)
+		ovl_set_flag(OVL_CONST_INO, inode);
+
 	/* Check for non-merge dir that may have whiteouts */
 	if (is_dir) {
 		if (((upperdentry && lowerdentry) || oip->numlower > 1) ||

fs/overlayfs/overlayfs.h

@@ -38,6 +38,8 @@ enum ovl_inode_flag {
 	OVL_WHITEOUTS,
 	OVL_INDEX,
 	OVL_UPPERDATA,
+	/* Inode number will remain constant over copy up. */
+	OVL_CONST_INO,
 };
 
 enum ovl_entry_flag {