SUNRPC: Replace the "__be32 *p" parameter to .pc_encode

The passed-in value of the "__be32 *p" parameter is now unused in
every server-side XDR encoder, and can be removed.

Note also that there is a line in each encoder that sets up a local
pointer to a struct xdr_stream. Passing that pointer from the
dispatcher instead saves one line per encoder function.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>