to offline a *free* page, the value of mce_bad_pages will be added, and
the page is set HWPoison flag, but it is still managed by page buddy
alocator.
$ cat /proc/meminfo | grep HardwareCorrupted
shows the value.
If we offline the same page, the value of mce_bad_pages will be added
*again*, this means the value is incorrect now. Assume the page is
still free during this short time.