scsi: target/core: Use the SECTOR_SHIFT constant

Instead of duplicating the SECTOR_SHIFT definition from <linux/blkdev.h>,
use it. This patch does not change any functionality.

Reviewed-by: Christoph Hellwig <hch@lst.de>
Cc: Nicholas Bellinger <nab@linux-iscsi.org>
Cc: Mike Christie <mchristi@redhat.com>
Cc: Hannes Reinecke <hare@suse.de>
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>