rbd: don't call rbd_osd_req_format_read() for !img_data requests

Accessing obj_request->img_request union field is only valid for object
requests associated with an image (i.e. if obj_request_img_data_test()
returns true).  rbd_osd_req_format_read() used to do more, but now it
just sets osd_req->snap_id.  Standalone and stat object requests always
go to the HEAD revision and are fine with CEPH_NOSNAP set by libceph,
so get around the invalid union field use by simply not calling
rbd_osd_req_format_read() in those places.

Reported-by: David Disseldorp <ddiss@suse.de>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
Reviewed-by: Alex Elder <elder@linaro.org>
Reviewed-by: David Disseldorp <ddiss@suse.de>

drivers/block/rbd.c

@@ -1951,11 +1951,10 @@ static void rbd_osd_req_callback(struct ceph_osd_request *osd_req)
 
 static void rbd_osd_req_format_read(struct rbd_obj_request *obj_request)
 {
-	struct rbd_img_request *img_request = obj_request->img_request;
 	struct ceph_osd_request *osd_req = obj_request->osd_req;
 
-	if (img_request)
-		osd_req->r_snapid = img_request->snap_id;
+	rbd_assert(obj_request_img_data_test(obj_request));
+	osd_req->r_snapid = obj_request->img_request->snap_id;
 }
 
 static void rbd_osd_req_format_write(struct rbd_obj_request *obj_request)
@@ -2937,8 +2936,6 @@ static int rbd_img_obj_exists_submit(struct rbd_obj_request *obj_request)
 	stat_request->page_count = page_count;
 	stat_request->callback = rbd_img_obj_exists_callback;
 
-	rbd_osd_req_format_read(stat_request);
-
 	rbd_obj_request_submit(stat_request);
 	return 0;
 
@@ -4034,7 +4031,6 @@ static int rbd_obj_method_sync(struct rbd_device *rbd_dev,
 	osd_req_op_cls_response_data_pages(obj_request->osd_req, 0,
 					obj_request->pages, inbound_size,
 					0, false, false);
-	rbd_osd_req_format_read(obj_request);
 
 	rbd_obj_request_submit(obj_request);
 	ret = rbd_obj_request_wait(obj_request);
@@ -4276,7 +4272,6 @@ static int rbd_obj_read_sync(struct rbd_device *rbd_dev,
 					obj_request->length,
 					obj_request->offset & ~PAGE_MASK,
 					false, false);
-	rbd_osd_req_format_read(obj_request);
 
 	rbd_obj_request_submit(obj_request);
 	ret = rbd_obj_request_wait(obj_request);