md/md-bitmap: remove parameter slot from bitmap_create()

All callers pass in '-1' for 'slot', hence it can be removed.

Link: https://lore.kernel.org/linux-raid/20250524061320.370630-6-yukuai1@huaweicloud.com


Signed-off-by: Yu Kuai <yukuai3@huawei.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Xiao Ni <xni@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Hannes Reinecke <hare@suse.de>