nvme-multipath: refactor nvme_mpath_add_disk

Pass anagrpid as second argument. This is prep patch that allows reusing
this function for supporting unknown command sets.

Signed-off-by: Joel Granados <j.granados@samsung.com>
Signed-off-by: Kanchan Joshi <joshi.k@samsung.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>