Add fill_device option

Add fill_device option. Causes fio to write until ENOSPC occurs (assuming
rw=write).

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
diff --git a/init.c b/init.c
index 83957cf..a503626 100644
--- a/init.c
+++ b/init.c
@@ -293,6 +293,9 @@
 		o->time_based = 0;
 	}
 
+	if (o->fill_device && !o->size)
+		o->size = ULONG_LONG_MAX;
+
 	return 0;
 }