block: disallow Persistent Reservation on partitions

Refuse Persistent Reservation operations on partitions as reservation
on partitions doesn't make sense.

Besides, introduce blkdev_pr_allowed() helper, where more policies could
be placed here later.

Signed-off-by: Jingbo Xu <jefflexu@linux.alibaba.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20230613084008.93795-2-jefflexu@linux.alibaba.com


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