mm,hwpoison: send SIGBUS to PF_MCE_EARLY processes on action required events

When a memory uncorrected error is triggered by process who accessed the
address with error, It's Action Required Case for only current process
which triggered this; This Action Required case means Action optional to
other process who share the same page.  Usually killing current process
will be sufficient, other processes sharing the same page will get be
signaled when they really touch the poisoned page.

But there is another scenario that other processes sharing the same page
want to be signaled early with PF_MCE_EARLY set.  In this case, we should
get them into kill list and signal BUS_MCEERR_AO to them.

So in this patch, task_early_kill will check current process if
force_early is set, and if not current,the code will fallback to
find_early_kill_thread() to check if there is PF_MCE_EARLY process who
cares the error.

In kill_proc(), BUS_MCEERR_AR is only send to current, other processes in
kill list will be signaled with BUS_MCEERR_AO.

Link: https://lkml.kernel.org/r/20210122132424.313c8f5f.yaoaili@kingsoft.com
Signed-off-by: Aili Yao <yaoaili@kingsoft.com>
Reviewed-by: Oscar Salvador <osalvador@suse.de>
Acked-by: Naoya Horiguchi <naoya.horiguchi@nec.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

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