Fix cpumask setting/getting on older kernels

Don't call into the affinity functions, unless the user has
specified a mask.

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
diff --git a/fio.c b/fio.c
index 0c297f7..6483e64 100644
--- a/fio.c
+++ b/fio.c
@@ -792,7 +792,7 @@
 	if (init_io_u(td))
 		goto err;
 
-	if (fio_setaffinity(td) == -1) {
+	if (td->o.cpumask_set && fio_setaffinity(td) == -1) {
 		td_verror(td, errno, "cpu_set_affinity");
 		goto err;
 	}