Add support for async IO verification offload

This adds support for setting up a number of IO verification offload
threads, instead of doing the offload inline. An option for controlling
the CPU affinity of those threads are always added.

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
diff --git a/os/os.h b/os/os.h
index dbf0957..10e796f 100644
--- a/os/os.h
+++ b/os/os.h
@@ -39,7 +39,7 @@
 #endif /* FIO_HAVE_FADVISE */
 
 #ifndef FIO_HAVE_CPU_AFFINITY
-#define fio_setaffinity(td)		(0)
+#define fio_setaffinity(pid, mask)	(0)
 #define fio_getaffinity(pid, mask)	do { } while (0)
 #define fio_cpu_clear(mask, cpu)	do { } while (0)
 #define fio_cpuset_exit(mask)		(-1)