md-cluster: add CHANGE_CAPACITY message type

The msg type CHANGE_CAPACITY is introduced to support
resize clustered raid in later patch, and it is sent
after all the nodes have the same sync_size, receiver
node just need to set new capacity once received this
msg.

Reviewed-by: NeilBrown <neilb@suse.com>
Signed-off-by: Guoqing Jiang <gqjiang@suse.com>
Signed-off-by: Shaohua Li <shli@fb.com>