nbd: fix assert() on qemu-nbd stop

From time to time qemu-nbd is crashing on the following assert:
    assert(state == TERMINATING);
    nbd_export_closed
    nbd_export_put
    main
and the state at the moment of the crash is evaluated to TERMINATE.

During shutdown process of the client the nbd_client_thread thread sends
SIGTERM signal and the main thread calls the nbd_client_closed callback.
If the SIGTERM callback will be executed after change the state to
TERMINATING, then the state will once again be TERMINATE.

To solve the issue, we must change the state to TERMINATE only if the state
is RUNNING. In the other case we are shutting down already.

Signed-off-by: Pavel Butsykin <pbutsykin@virtuozzo.com>
Signed-off-by: Denis V. Lunev <den@openvz.org>
CC: Paolo Bonzini <pbonzini@redhat.com>
Message-id: 1460629215-11567-1-git-send-email-den@openvz.org
Signed-off-by: Max Reitz <mreitz@redhat.com>
