Fix issue with random offset being off if io_size != real_file_size

get_next_free_block() would generate offset that were larger than the
requested size, either causing failures or just causing the test
to exit prematurely.

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
diff --git a/io_u.c b/io_u.c
index d92d940..c0beafd 100644
--- a/io_u.c
+++ b/io_u.c
@@ -113,7 +113,8 @@
 
 	i = f->last_free_lookup;
 	*b = (i * BLOCKS_PER_MAP);
-	while ((*b) * min_bs < f->real_file_size) {
+	while ((*b) * min_bs < f->real_file_size &&
+		(*b) * min_bs < f->io_size) {
 		if (f->file_map[i] != (unsigned int) -1) {
 			*b += ffz(f->file_map[i]);
 			if (*b > last_block(td, f, ddir))