scsi: target: Add callout to configure UNMAP settings

Add a callout to configure a backend's UNMAP settings. This will be used to
allow userspace to configure UNMAP after the initial device setup, similar
to how we can set up the other attributes post device configuration.

Link: https://lore.kernel.org/r/20220628200230.15052-3-michael.christie@oracle.com
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Mike Christie <michael.christie@oracle.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>