md-cluster: make md-cluster also can work when compiled into kernel

The md-cluster is compiled as module by default,
if it is compiled by built-in way, then we can't
make md-cluster works.

[64782.630008] md/raid1:md127: active with 2 out of 2 mirrors
[64782.630528] md-cluster module not found.
[64782.630530] md127: Could not setup cluster service (-2)

Fixes: edb39c9d ("Introduce md_cluster_operations to handle cluster functions")
Cc: stable@vger.kernel.org (v4.1+)
Reported-by: Marc Smith <marc.smith@mcc.edu>
Reviewed-by: NeilBrown <neilb@suse.com>
Signed-off-by: Guoqing Jiang <gqjiang@suse.com>
Signed-off-by: Shaohua Li <shli@fb.com>