ioprio: allow sys_ioprio_set() value of 0 to reset ioprio setting

Normally io priorities follow the CPU nice, unless a specific scheduling
class has been set. Once that is set, there's no way to reset the
behaviour to 'none' so that it follows CPU nice again.

Currently passing in 0 as the ioprio class/value will return -1/EINVAL,
change that to allow resetting of a set scheduling class.
Signed-off-by: NJens Axboe <jens.axboe@oracle.com>

fs/ioprio.c

@@ -78,6 +78,10 @@ asmlinkage long sys_ioprio_set(int which, int who, int ioprio)
 			if (!capable(CAP_SYS_ADMIN))
 				return -EPERM;
 			break;
+		case IOPRIO_CLASS_NONE:
+			if (data)
+				return -EINVAL;
+			break;
 		default:
 			return -EINVAL;
 	}