ovl: add accessor for ofs->upper_mnt

Next patch will remove ofs->upper_mnt, so add an accessor function for this
field.

Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>

fs/overlayfs/namei.c

@@ -489,7 +489,7 @@ struct dentry *ovl_index_upper(struct ovl_fs *ofs, struct dentry *index)
 	if (IS_ERR_OR_NULL(fh))
 		return ERR_CAST(fh);
 
-	upper = ovl_decode_real_fh(fh, ofs->upper_mnt, true);
+	upper = ovl_decode_real_fh(fh, ovl_upper_mnt(ofs), true);
 	kfree(fh);
 
 	if (IS_ERR_OR_NULL(upper))