scsi: target: Add file configure_unmap callout

Move file's UNMAP setup code to a configure_unmap callout.

Link: https://lore.kernel.org/r/20220628200230.15052-5-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>