staging: erofs: replace BUG_ON with DBG_BUGON in data.c

This patch replace BUG_ON with DBG_BUGON in data.c, and add necessary
error handler.

Signed-off-by: Chen Gong <gongchen4@huawei.com>
Reviewed-by: Gao Xiang <gaoxiang25@huawei.com>
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Gao Xiang <gaoxiang25@huawei.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>