dm zoned: properly handle backing device failure

dm zoned: properly handle backing device failure

dm-zoned is observed to lock up or livelock in case of hardware
failure or some misconfiguration of the backing zoned device.

This patch adds a new dm-zoned target function that checks the status of
the backing device. If the request queue of the backing device is found
to be in dying state or the SCSI backing device enters offline state,
the health check code sets a dm-zoned target flag prompting all further
incoming I/O to be rejected. In order to detect backing device failures
timely, this new function is called in the request mapping path, at the
beginning of every reclaim run and before performing any metadata I/O.

The proper way out of this situation is to do

dmsetup remove <dm-zoned target>

and recreate the target when the problem with the backing device
is resolved.

Fixes: 3b1a94c88b79 ("dm zoned: drive-managed zoned block device target")
Cc: stable@vger.kernel.org
Signed-off-by: Dmitry Fomichev <dmitry.fomichev@wdc.com>
Reviewed-by: Damien Le Moal <damien.lemoal@wdc.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>