sunrpc/auth_gss: fix decoder callback prototypes

Declare the p_decode callbacks with the proper prototype instead of
casting to kxdrdproc_t and losing all type safety.

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