Confusion between mutex and semaphore naming

There's a semaphore implementation and option coming, so rename the
bits.

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
diff --git a/fio.h b/fio.h
index 0c5b648..d7875b0 100644
--- a/fio.h
+++ b/fio.h
@@ -563,7 +563,7 @@
 	unsigned long long io_skip_bytes;
 	unsigned long long this_io_bytes[2];
 	unsigned long long zone_bytes;
-	struct fio_sem *mutex;
+	struct fio_mutex *mutex;
 
 	/*
 	 * State for random io, a bitmap of blocks done vs not done