md/md-bitmap: delay registration of bitmap_ops until creating bitmap

md/md-bitmap: delay registration of bitmap_ops until creating bitmap

Currently bitmap_ops is registered while allocating mddev, this is fine
when there is only one bitmap_ops.

Delay setting bitmap_ops until creating bitmap, so that user can choose
which bitmap to use before running the array.

Link: https://lore.kernel.org/linux-raid/20250721171557.34587-7-yukuai@kernel.org
Signed-off-by: Yu Kuai <yukuai3@huawei.com>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Reviewed-by: Li Nan <linan122@huawei.com>
Reviewed-by: Xiao Ni <xni@redhat.com>