ubi: Split io_failures into write_failure and erase_failure

hulk inclusion
category: feature
bugzilla: https://gitee.com/openeuler/kernel/issues/I8QZWQ


CVE: NA

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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>