Extend --readonly

- Never allow open of a data file with writeable bits sets, and that
  includes extend as well.

- Skip any writes in replay logs (iolog/iolog2/blktrace)

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
diff --git a/fio.h b/fio.h
index 5177795..20d1408 100644
--- a/fio.h
+++ b/fio.h
@@ -661,6 +661,7 @@
 extern int temp_stall_ts;
 extern unsigned long long mlock_size;
 extern unsigned long page_mask, page_size;
+extern int read_only;
 
 extern struct thread_data *threads;