md-cluster: complete all write requests before adding suspend_info

process_suspend_info - which handles the RESYNCING request - must not
reply until all writes which were initiated before the request arrived,
have completed.

As a by-product, all process_* functions now take mddev as their
first arguement making it uniform.

Signed-off-by: Goldwyn Rodrigues <rgoldwyn@suse.com>
Signed-off-by: NeilBrown <neilb@suse.com>