fix parse param bug and set default when overflow

[guodashun]

1. In this patch, we check if the value is overflow before parsing the value with its unit. we replaced sscanf with strtoul cause the strtoul has clear errno ERANGE for overflow case.
2. In this patch, we deleted the wrong logic unit_matched=0. When we set the param with a high level unit (e.g. PAGE_CE_THRESHOLD=1m), the real value we expect is 1000000(1*1000*1000), but now we only match the unit 'm' and multiply 1000 once, so the value we get is 1000 rather than 1000000. In this patch we fix the bug.
3. When the value is overflow, the sscanf will produce Undefined Behavior (https://man7.org/linux/man-pages/man3/sscanf.3.html#BUGS). The final value after being truncated is confusing. So in this patch we will set to default value when the value is overflow.