rbd: don't leak rbd_req for rbd_req_sync_notify_ack()

When rbd_req_sync_notify_ack() calls rbd_do_request() it supplies
rbd_simple_req_cb() as its callback function.  Because the callback
is supplied, an rbd_req structure gets allocated and populated so it
can be used by the callback.  However rbd_simple_req_cb() is not
freeing (or even using) the rbd_req structure, so it's getting
leaked.

Since rbd_simple_req_cb() has no need for the rbd_req structure,
just avoid allocating one for this case.  Of the three calls to
rbd_do_request(), only the one from rbd_do_op() needs the rbd_req
structure, and that call can be distinguished from the other two
because it supplies a non-null rbd_collection pointer.

So fix this leak by only allocating the rbd_req structure if a
non-null "coll" value is provided to rbd_do_request().

Signed-off-by: Alex Elder <elder@inktank.com>
Reviewed-by: Josh Durgin <josh.durgin@inktank.com>

drivers/block/rbd.c

@@ -1135,7 +1135,7 @@ static int rbd_do_request(struct request *rq,
 		bio_get(osd_req->r_bio);
 	}
 
-	if (rbd_cb) {
+	if (coll) {
 		ret = -ENOMEM;
 		rbd_req = kmalloc(sizeof(*rbd_req), GFP_NOIO);
 		if (!rbd_req)
@@ -1146,7 +1146,7 @@ static int rbd_do_request(struct request *rq,
 		rbd_req->pages = pages;
 		rbd_req->len = len;
 		rbd_req->coll = coll;
-		rbd_req->coll_index = coll ? coll_index : 0;
+		rbd_req->coll_index = coll_index;
 	}
 
 	osd_req->r_callback = rbd_cb;