sunrpc: properly type pc_encode callbacks

Drop the resp argument as it can trivially be derived from the rqstp
argument.  With that all functions now have the same prototype, and we
can remove the unsafe casting to kxdrproc_t.

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