nvme: replace the GENHD_FL_UP check in nvme_mpath_shutdown_disk

Use the nvme-internal NVME_NSHEAD_DISK_LIVE flag instead of abusing
the block layer state.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20210809064028.1198327-5-hch@lst.de


Signed-off-by: Jens Axboe <axboe@kernel.dk>