target: Fail WRITE_SAME w/ UNMAP=1 when emulate_tpws=0

This patch adds a check within sbc_setup_write_same() to fail a
WRITE_SAME w/ UNMAP=1 op, if the backend device has emulate_tpws
disabled.

Cc: Martin Petersen <martin.petersen@oracle.com>
Cc: Christoph Hellwig <hch@lst.de>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>

drivers/target/target_core_sbc.c

@@ -293,6 +293,11 @@ sbc_setup_write_same(struct se_cmd *cmd, unsigned char *flags, struct sbc_ops *o
 		if (!ops->execute_write_same_unmap)
 			return TCM_UNSUPPORTED_SCSI_OPCODE;
 
+		if (!dev->dev_attrib.emulate_tpws) {
+			pr_err("Got WRITE_SAME w/ UNMAP=1, but backend device"
+			       " has emulate_tpws disabled\n");
+			return TCM_UNSUPPORTED_SCSI_OPCODE;
+		}
 		cmd->execute_cmd = ops->execute_write_same_unmap;
 		return 0;
 	}