scsi: target: fix unmap_zeroes_data boolean initialisation

The LIO unmap_zeroes_data device attribute is mapped to the LBPRZ flag in
the READ CAPACITY(16) and Thin Provisioning VPD INQUIRY responses.

The unmap_zeroes_data attribute is exposed via configfs, where any write
value is correctly validated via strtobool(). However, when initialised via
target_configure_unmap_from_queue() it takes the value of the device's
max_write_zeroes_sectors queue limit, which is non-boolean.

A non-boolean value can be read from configfs, but attempting to write the
same value back results in -EINVAL, causing problems for configuration
utilities such as targetcli.

Link: https://marc.info/?l=target-devel&m=158213354011309


Fixes: 2237498f ("target/iblock: Convert WRITE_SAME to blkdev_issue_zeroout")
Reviewed-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: David Disseldorp <ddiss@suse.de>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>