rbd nbd: Add cond to signal nbd disconnection

rbd nbd: Add cond to signal nbd disconnection

With the ioctl interface we know when a disconnect has been executed
because the NBD_DO_IO ioctl will return. The netlink interface returns
after the nl connect cmd call has completed, so this patch adds a cond
that signals when a nl disconnect/NBD_DISCONNECT has been executed and
the server thread has handled the resulting NBD_CMD_DISC.

Signed-off-by: Mike Christie <mchristi@redhat.com>