block: Cleanup set_capacity()/bdev_set_nr_sectors()

The code for setting a block device capacity (bd_nr_sectors field of
struct block_device) is duplicated in set_capacity() and
bdev_set_nr_sectors(). Clean this up by making bdev_set_nr_sectors()
a block layer internal function defined in block/bdev.c instead of
having this function statically defined in block/partitions/core.c.
With this change, set_capacity() implementation can be simplified to
only calling bdev_set_nr_sectors().

Signed-off-by: Damien Le Moal <dlemoal@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20230424131318.79935-1-dlemoal@kernel.org
Signed-off-by: Jens Axboe <axboe@kernel.dk>