dm-error is used in several test cases in the xfstests test suite to
check the handling of IO errors in file systems. However, with several
file systems getting native support for zoned block devices (e.g.
btrfs and f2fs), dm-error's lack of zoned block device support creates
problems as the file system attempts executing zone commands (e.g. a
zone append operation) against a dm-error non-zoned block device,
which causes various issues in the block layer (e.g. WARN_ON
triggers).

This commit adds supports for zoned block devices to dm-error, allowing
a DM device table containing an error target to be exposed as a zoned
block device (if all targets have a compatible zoned model support and
mapping). This is done as follows:
1) Allow passing 2 arguments to an error target, similar to dm-linear:
   a backing device and a start sector. These arguments are optional and
   dm-error retains its characteristics if the arguments are not
   specified.
2) Implement the iterate_devices method so that dm-core can normally
   check the zone support and restrictions (e.g. zone alignment of the
   targets). When the backing device arguments are not specified, the
   iterate_devices method never calls the fn() argument.
When no backing device is specified, as before, we assume that the DM
device is not zoned. When the backing device arguments are specified,
the zoned model of the DM device will depend on the backing device
type:
 - If the backing device is zoned and its model and mapping is
   compatible with other targets of the device, the resulting device
   will be zoned, with the dm-error mapped portion always returning
   errors (similar to the default non-zoned case).
 - If the backing device is not zoned, then the DM device will not be
   either.

This zone support for dm-error requires the definition of a functional
report_zones operation so that dm_revalidate_zones() can operate
correctly and resources for emulating zone append operations
initialized. This is necessary for cases where dm-error is used to
partially map a device and have an overall correct handling of zone
append. This means that dm-error does not fail report zones operations.

Two changes that are not obvious are included to avoid issues:
1) dm_table_supports_zoned_model() is changed to directly check if
   the backing device of a wildcard target (= dm-error target) is
   zoned. Otherwise, we wouldn't be able to catch the invalid setup of
   dm-error without a backing device (non zoned case) being combined
   with zoned targets.
2) dm_table_supports_dax() is modified to return false if the wildcard
   target is found. Otherwise, when dm-error is set without a backing
   device, we end up with a NULL pointer dereference in
   set_dax_synchronous (dax_dev is NULL). This is consistent with the
   current behavior because dm_table_supports_dax() always returned
   false for targets that do not define the iterate_devices method.
Signed-off-by: Damien Le Moal <dlemoal@kernel.org>
Tested-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Mike Snitzer <snitzer@kernel.org>