sheepdog: fix co_recv coroutine context

sheepdog: fix co_recv coroutine context

The co_recv coroutine has two things that will try to enter it:

  1. The select(2) read callback on the sheepdog socket.
  2. The aio_add_request() blocking operations, including a coroutine
     mutex.

This patch fixes it by setting NULL to co_recv before sending data.

In future, we should make the sheepdog driver fully coroutine-based
and simplify request handling.

Signed-off-by: MORITA Kazutaka <morita.kazutaka@lab.ntt.co.jp>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>