Use td_io_get_file_size() instead of calling ->get_file_size()

It may be NULL on some engines, causing a crash.

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
diff --git a/filesetup.c b/filesetup.c
index 68516b9..af23191 100644
--- a/filesetup.c
+++ b/filesetup.c
@@ -397,7 +397,7 @@
 		dprint(FD_FILE, "get file size for %p/%d/%p\n", f, i,
 								f->file_name);
 
-		if (td->io_ops->get_file_size(td, f)) {
+		if (td_io_get_file_size(td, f)) {
 			if (td->error != ENOENT) {
 				log_err("%s\n", td->verror);
 				err = 1;