mm,hwpoison: check mm when killing accessing process

The GHES code calls memory_failure_queue() from IRQ context to queue work
into workqueue and schedule it on the current CPU.  Then the work is
processed in memory_failure_work_func() by kworker and calls
memory_failure().

When a page is already poisoned, commit a3f5d80ea401 ("mm,hwpoison: send
SIGBUS with error virutal address") make memory_failure() call
kill_accessing_process() that:

    - holds mmap locking of current->mm
    - does pagetable walk to find the error virtual address
    - and sends SIGBUS to the current process with error info.

However, the mm of kworker is not valid, resulting in a null-pointer
dereference.  So check mm when killing the accessing process.

[[email protected]: remove unrelated whitespace alteration]
Link: https://lkml.kernel.org/r/[email protected]
Fixes: a3f5d80ea401 ("mm,hwpoison: send SIGBUS with error virutal address")
Signed-off-by: Shuai Xue <[email protected]>
Reviewed-by: Miaohe Lin <[email protected]>
Acked-by: Naoya Horiguchi <[email protected]>
Cc: Huang Ying <[email protected]>
Cc: Baolin Wang <[email protected]>
Cc: Bixuan Cui <[email protected]>
Cc: <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

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