s390/pfault: fix task state race

When setting the current task state to TASK_UNINTERRUPTIBLE this can
race with a different cpu. The other cpu could set the task state after
it inspected it (while it was still TASK_RUNNING) to TASK_RUNNING which
would change the state from TASK_UNINTERRUPTIBLE to TASK_RUNNING again.

This race was always present in the pfault interrupt code but didn't
cause anything harmful before commit f2db2e6c "[S390] pfault: cpu hotplug
vs missing completion interrupts" which relied on the fact that after
setting the task state to TASK_UNINTERRUPTIBLE the task would really
sleep.
Since this is not necessarily the case the result may be a list corruption
of the pfault_list or, as observed, a use-after-free bug while trying to
access the task_struct of a task which terminated itself already.

To fix this, we need to get a reference of the affected task when receiving
the initial pfault interrupt and add special handling if we receive yet
another initial pfault interrupt when the task is already enqueued in the
pfault list.

Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Reviewed-by: Martin Schwidefsky <schwidefsky@de.ibm.com>
Cc: <stable@vger.kernel.org> # needed for v3.0 and newer
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>