block/nfs: fix potential segfault on early callback it will happen in the future that the callback of a libnfs call directly invokes the callback. In this case we end up in a segfault because the NFSRPC is gone when we the BH is scheduled. Signed-off-by: Peter Lieven <pl@kamp.de> Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>