xprtrdma: Fix use of xdr_stream_encode_item_{present, absent}

These new helpers do not return 0 on success, they return the
encoded size. Thus they are not a drop-in replacement for the
old helpers.

Fixes: 5c266df5 ("SUNRPC: Add encoders for list item discriminators")
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>