md/raid10: fix overflow in safe_delay_store

hulk inclusion
category: bugfix
bugzilla: 188284, https://gitee.com/openeuler/kernel/issues/I6ECB2


CVE: NA

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There is no input check when echo md/safe_mode_delay, and overflow will
occur. There is risk of overflow in strict_strtoul_scaled(), too. Fixed
it by using kstrtoul instead of parsing word one by one.

Signed-off-by: Li Nan <linan122@huawei.com>
Reviewed-by: Hou Tao <houtao1@huawei.com>
Signed-off-by: Jialin Zhang <zhangjialin11@huawei.com>