md: introduce struct md_submodule_head and APIs

Prepare to unify registration and unregistration of md personalities
and md-cluster, also prepare for add kconfig for md-bitmap.

Link: https://lore.kernel.org/linux-raid/20250215092225.2427977-4-yukuai1@huaweicloud.com


Signed-off-by: Yu Kuai <yukuai3@huawei.com>