SUNRPC: Remove pointer type casts from xdr_get_next_encode_buffer()

To make the code easier to read, remove visual clutter by changing
the declared type of @p.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Reviewed-by: NeilBrown <neilb@suse.de>
Reviewed-by: J. Bruce Fields <bfields@fieldses.org>