sunrpc: properly type argument to kxdrdproc_t

Pass struct rpc_request as the first argument instead of an untyped blob.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Jeff Layton <jlayton@redhat.com>
Acked-by: Trond Myklebust <trond.myklebust@primarydata.com>