dm verity: skip verity work on I/O errors when system is shutting down

If emergency system shutdown is called, like by thermal shutdown,
dm device could be alive when the block device couldn't process
I/O requests anymore. In this status, the handling of I/O errors
by new dm I/O requests or by those already in-flight can lead to
a verity corruption state, which is misjudgment.
So, skip verity work for I/O error when system is shutting down.

Change-Id: I7b2e79283bb5810cefe688bf43b9ae97030ce917
Reviewed-by: Sami Tolvanen <samitolvanen@google.com>
Signed-off-by: Hyeongseok Kim <hyeongseok@gmail.com>
Patch-mainline: dm-devel @ 12/03/20, 00:46
Signed-off-by: Ravi Kumar Siddojigari <rsiddoji@codeaurora.org>
Signed-off-by: Srinivasarao P <spathi@codeaurora.org>