block: add a disk_has_partscan helper

Add a helper to check if partition scanning is enabled instead of
open coding the check in a few places.  This now always checks for
the hidden flag even if all but one of the callers are never reachable
for hidden gendisks.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20240502130033.1958492-2-hch@lst.de


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