scsi: retry MODE SENSE on unit attention

The 'sd' driver is calling scsi_mode_sense() to figure out
internal details. But scsi_mode_sense() never checks for
any pending unit attentions, so we're getting annoying error
messages like:

MODE SENSE: unimplemented page/subpage: 0x00/0x00

and a possible wrong decision for device cache handling.
Reviewed-by: NEwan Milne <emilne@redhat.com>
Signed-off-by: NHannes Reinecke <hare@suse.de>
Signed-off-by: NJames Bottomley <JBottomley@Odin.com>

drivers/scsi/scsi_lib.c

@@ -2423,7 +2423,7 @@ scsi_mode_sense(struct scsi_device *sdev, int dbd, int modepage,
 	unsigned char cmd[12];
 	int use_10_for_ms;
 	int header_length;
-	int result;
+	int result, retry_count = retries;
 	struct scsi_sense_hdr my_sshdr;
 
 	memset(data, 0, sizeof(*data));
@@ -2502,6 +2502,11 @@ scsi_mode_sense(struct scsi_device *sdev, int dbd, int modepage,
 			data->block_descriptor_length = buffer[3];
 		}
 		data->header_length = header_length;
+	} else if ((status_byte(result) == CHECK_CONDITION) &&
+		   scsi_sense_valid(sshdr) &&
+		   sshdr->sense_key == UNIT_ATTENTION && retry_count) {
+		retry_count--;
+		goto retry;
 	}
 
 	return result;