Just like drivers should update current_ctrl and not block->ctrl,
the same happens to buffers. This only applies to vmalloc, though.

Bug introduced by my commit 6a7378a5

:
      zio-sys: delay copying current_ctrl to block->ctrl

Signed-off-by: Alessandro Rubini <rubini@gnudd.com>
Acked-by: Federico Vaga <federico.vaga@gmail.com>