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/fio.h b/fio.h
index b7b25b2..7c39be3 100644
--- a/fio.h
+++ b/fio.h
@@ -393,6 +393,7 @@
 	unsigned int iodepth_batch;
 
 	unsigned long long size;
+	unsigned int fill_device;
 	unsigned long long file_size_low;
 	unsigned long long file_size_high;
 	unsigned long long start_offset;