Now that no fast path lookups in the partition table are left, there is
no point in micro-optimizing the data structure for it.  Just use a bog
standard xarray.
Signed-off-by: NChristoph Hellwig <hch@lst.de>
Acked-by: NTejun Heo <tj@kernel.org>
Signed-off-by: NJens Axboe <axboe@kernel.dk>