md: switch md-cluster to use md_submodle_head

To make code cleaner, and prepare to add kconfig for bitmap.

Also remove the unsed global variables pers_lock, md_cluster_ops and
md_cluster_mod, and exported symbols register_md_cluster_operations(),
unregister_md_cluster_operations() and md_cluster_ops.

Link: https://lore.kernel.org/linux-raid/20250215092225.2427977-8-yukuai1@huaweicloud.com


Signed-off-by: Yu Kuai <yukuai3@huawei.com>
Reviewed-by: Su Yue <glass.su@suse.com>