block/nbd: make nbd_co_establish_connection_cancel() bs-independent

nbd_co_establish_connection_cancel() actually needs only pointer to
NBDConnectThread. So, make it clean.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Reviewed-by: Roman Kagan <rvkagan@yandex-team.ru>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <20210610100802.5888-14-vsementsov@virtuozzo.com>
Signed-off-by: Eric Blake <eblake@redhat.com>