target: make the ->get_cdb method optional

The most commonly used file, iblock and rd backends have no use for
a per-task CDB and thus don't need a method to copy it into their
otherwise unused CDB fields.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>
diff --git a/drivers/target/target_core_file.h b/drivers/target/target_core_file.h
index daebd71..59e6e73 100644
--- a/drivers/target/target_core_file.h
+++ b/drivers/target/target_core_file.h
@@ -14,9 +14,7 @@
 
 struct fd_request {
 	struct se_task	fd_task;
-	/* SCSI CDB from iSCSI Command PDU */
-	unsigned char	fd_scsi_cdb[TCM_MAX_COMMAND_SIZE];
-} ____cacheline_aligned;
+};
 
 #define FBDF_HAS_PATH		0x01
 #define FBDF_HAS_SIZE		0x02