ubi: Split io_failures into write_failure and erase_failure

The emulate_io_failures debugfs entry controls both write
failure and erase failure. This patch split io_failures
to write_failure and erase_failure.

Signed-off-by: ZhaoLong Wang <wangzhaolong1@huawei.com>
Reviewed-by: Zhihao Cheng <chengzhihao1@huawei.com>
Signed-off-by: Richard Weinberger <richard@nod.at>