Label elements of rx structures

Label elements of rx structures

Label all of the assignments to the xdr_ops structure. This allows
the rec, stdio and mem variants to continue to function, even when
the ordering of the elements has changed in the header.

Change-Id: I006bbbdb8923dbb72a97fde931a1b23e303375c2
Reviewed-on: http://gerrit.openafs.org/1126
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>