md: don't export md_cluster_ops

Add a new field 'cluster_ops' and initialize it md_setup_cluster(), so
that the gloable variable 'md_cluter_ops' doesn't need to be exported.
Also prepare to switch md-cluster to use md_submod_head.

Link: https://lore.kernel.org/linux-raid/20250215092225.2427977-7-yukuai1@huaweicloud.com


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