__usec_sleep() really just spins, rename it

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
diff --git a/fio.c b/fio.c
index 3944ce7..a11f2bb 100644
--- a/fio.c
+++ b/fio.c
@@ -673,7 +673,7 @@
 				int left;
 
 				if (td->o.thinktime_spin)
-					__usec_sleep(td->o.thinktime_spin);
+					usec_spin(td->o.thinktime_spin);
 
 				left = td->o.thinktime - td->o.thinktime_spin;
 				if (left)