md: refactor action_store() for 'idle' and 'frozen'

Prepare to handle 'idle' and 'frozen' differently to fix a deadlock, there
are no functional changes except that MD_RECOVERY_RUNNING is checked
again after 'reconfig_mutex' is held.

Signed-off-by: Yu Kuai <yukuai3@huawei.com>
Signed-off-by: Song Liu <song@kernel.org>
Link: https://lore.kernel.org/r/20230529132037.2124527-3-yukuai1@huaweicloud.com