md: switch personalities to use md_submodule_head

Remove the global list 'pers_list', and switch to use md_submodule_head,
which is managed by xarry. Prepare to unify registration and unregistration
for all sub modules.

Link: https://lore.kernel.org/linux-raid/20250215092225.2427977-5-yukuai1@huaweicloud.com


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