ovl: fix open in stacked overlay

If two overlayfs filesystems are stacked on top of each other, then we need
recursion in ovl_d_select_inode().

I guess d_backing_inode() is supposed to do that.  But currently it doesn't
and that functionality is open coded in vfs_open().  This is now copied
into ovl_d_select_inode() to fix this regression.
Reported-by: Alban Crequy <alban.crequy@gmail.com>
Signed-off-by: Miklos Szeredi <miklos@szeredi.hu>
Fixes: 4bacc9c9 ("overlayfs: Make f_path always point to the overlay...")
Cc: David Howells <dhowells@redhat.com>
Cc: <stable@vger.kernel.org> # v4.2+

fs/overlayfs/inode.c

@@ -363,6 +363,9 @@ struct inode *ovl_d_select_inode(struct dentry *dentry, unsigned file_flags)
 		ovl_path_upper(dentry, &realpath);
 	}
 
+	if (realpath.dentry->d_flags & DCACHE_OP_SELECT_INODE)
+		return realpath.dentry->d_op->d_select_inode(realpath.dentry, file_flags);
+
 	return d_backing_inode(realpath.dentry);
 }