md/md-bitmap: add a new sysfs api bitmap_type

md/md-bitmap: add a new sysfs api bitmap_type

The api will be used by mdadm to set bitmap_type while creating new array
or assembling array, prepare to add a new bitmap.

Currently available options are:

cat /sys/block/md0/md/bitmap_type
none [bitmap]

Link: https://lore.kernel.org/linux-raid/20250829080426.1441678-6-yukuai1@huaweicloud.com
Signed-off-by: Yu Kuai <yukuai3@huawei.com>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Reviewed-by: Xiao Ni <xni@redhat.com>
Reviewed-by: Li Nan <linan122@huawei.com>