virtio_blk: add default case to cmd type switch

On compilation, gcc correctly detects that we do not handle
all types:

In function ‘blk_done’:
warning: enumeration value ‘REQ_TYPE_FS’ not handled in switch
warning: enumeration value ‘REQ_TYPE_SENSE’ not handled in switch
warning: enumeration value ‘REQ_TYPE_PM_SUSPEND’ not handled in switch
warning: enumeration value ‘REQ_TYPE_PM_RESUME’ not handled in switch
warning: enumeration value ‘REQ_TYPE_PM_SHUTDOWN’ not handled in switch
warning: enumeration value ‘REQ_TYPE_LINUX_BLOCK’ not handled in switch
warning: enumeration value ‘REQ_TYPE_ATA_TASKFILE’ not handled in switch
warning: enumeration value ‘REQ_TYPE_ATA_PC’ not handled in switch

which is a bit pointless since this is at the end of the request
processessing. Add a default case that just breaks out.
Signed-off-by: Jens Axboe <jaxboe@fusionio.com>

drivers/block/virtio_blk.c

@@ -74,6 +74,8 @@ static void blk_done(struct virtqueue *vq)
 		case REQ_TYPE_SPECIAL:
 			vbr->req->errors = (error != 0);
 			break;
+		default:
+			break;
 		}
 
 		__blk_end_request_all(vbr->req, error);