nvdimm/pmem: use add_disk() error handling

Now that device_add_disk() supports returning an error, use
that. We must unwind alloc_dax() on error.

Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Luis Chamberlain <mcgrof@kernel.org>
Link: https://lore.kernel.org/r/20211103230437.1639990-7-mcgrof@kernel.org


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