Seperate ioprio value from setting

Newer kernels allow passing a value of '0' for resetting ioprio,
and our check for if (td->ioprio) doesn't allow passing a value of
0 into sys_ioprio_set(). So seperate the setting from the value by
adding a ->ioprio_set bool.

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
diff --git a/fio.h b/fio.h
index 208f742..5ca2ad3 100644
--- a/fio.h
+++ b/fio.h
@@ -522,6 +522,7 @@
 	volatile int terminate;
 	volatile int runstate;
 	unsigned int ioprio;
+	unsigned int ioprio_set;
 	unsigned int last_was_sync;
 
 	char *mmapfile;