test/librbd/test_notify.py: effect post object map rebuild assert

test/librbd/test_notify.py: effect post object map rebuild assert

Instead of just optionally skipping update_features test, commit
9c0b239d70cd ("qa/upgrade: conditionally disable update_features
tests") moved it after rebuild_object_map test.  This isn't right
because update_features test invalidates the object map as a side
effect and rebuild_object_map test is what makes it valid again:

  assert((image.flags() & RBD_FLAG_OBJECT_MAP_INVALID) == 0)

Let's make this assert effective, at least when update_features
test isn't skipped.

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>