engines: rename option specific pad from 'td' to 'pad'

Except the net engine, they all just use it as padding to avoid
having a zero offset member. The latter makes the parser think
the offset isn't set, so not valid.

Signed-off-by: Jens Axboe <axboe@fb.com>
diff --git a/engines/libaio.c b/engines/libaio.c
index f7e2f89..d4f4830 100644
--- a/engines/libaio.c
+++ b/engines/libaio.c
@@ -37,7 +37,7 @@
 };
 
 struct libaio_options {
-	struct thread_data *td;
+	void *pad;
 	unsigned int userspace_reap;
 };