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.h b/fio.h
index 2717f5d..9a43c6f 100644
--- a/fio.h
+++ b/fio.h
@@ -406,6 +406,7 @@
 	unsigned int new_group;
 	unsigned int numjobs;
 	os_cpu_mask_t cpumask;
+	unsigned int cpumask_set;
 	unsigned int iolog;
 	unsigned int rwmixcycle;
 	unsigned int rwmix[2];