engines/libzbc: enable block backend

engines/libzbc: enable block backend

When opening a device, the current version of libzbc ioengine instructs
libzbc to only try SCSI and ATA backends for scanning the drive. This
prevents opening null_blk devices that fail to be accepted by the both
above mentioned backends and require the block backend to be enabled.

Set the appropriate flag to enable the block backend in zbc_open()
libzbc call.

Signed-off-by: Dmitry Fomichev <dmitry.fomichev@wdc.com>
Reviewed-by: Shin'ichiro Kawasaki <shinichiro.kawasaki@wdc.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>