md: merge common code into find_pers()

- pers_lock() are held and released from caller
- try_module_get() is called from caller
- error message from caller

Merge above code into find_pers(), and rename it to get_pers(), also
add a wrapper to module_put() as put_pers().

Link: https://lore.kernel.org/linux-raid/20250215092225.2427977-2-yukuai1@huaweicloud.com


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