md: only include md-cluster.h if necessary

md-cluster is only supportted by raid1 and raid10, there is no need to
include md-cluster.h for other personalities.

Also move APIs that is only used in md-cluster.c from md.h to
md-cluster.h.

Link: https://lore.kernel.org/linux-raid/20250215092225.2427977-3-yukuai1@huaweicloud.com


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