rxgen: Don't use size_t in struct rx_opaque with XDR

rxgen: Don't use size_t in struct rx_opaque with XDR

OpenAFS's XDR doesn't support size_t at this time.  For now, use a
temporary stack variable to avoid 32/64-bit issues and copy back the
returned value upon success.

Change-Id: Ia3dd8abd665a19e04aa611f940728d088a8f87b7
Reviewed-on: https://gerrit.openafs.org/12115
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Michael Meffie <mmeffie@sinenomine.net>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>