Constify 'td' in a few functions

Signed-off-by: Jens Axboe <axboe@fb.com>
diff --git a/fio.h b/fio.h
index 136b430..f981739 100644
--- a/fio.h
+++ b/fio.h
@@ -406,7 +406,7 @@
 
 extern struct thread_data *threads;
 
-static inline void fio_ro_check(struct thread_data *td, struct io_u *io_u)
+static inline void fio_ro_check(const struct thread_data *td, struct io_u *io_u)
 {
 	assert(!(io_u->ddir == DDIR_WRITE && !td_write(td)));
 }