md-cluster: split recover_slot for future code reuse

Make recover_slot as a wraper to __recover_slot, since the
logic of __recover_slot can be reused for the condition
when other nodes need to take over the resync job.

Signed-off-by: Guoqing Jiang <gqjiang@suse.com>
Signed-off-by: NeilBrown <neilb@suse.com>