nvme-rdma: Make sure to shutdown the controller if we can

Relying on ctrl state in nvme_rdma_shutdown_ctrl is wrong because
it will never be NVME_CTRL_LIVE (delete_ctrl or reset_ctrl invoked it).

Instead, check that the admin queue is connected. Note that it is safe
because we can never see a copmeting thread trying to destroy the admin
queue (reset or delete controller).

Signed-off-by: Sagi Grimberg <sagi@grimberg.me>
Reviewed-by: Christoph Hellwig <hch@lst.de>