rx: Named values for securityObjectStats types

Stop using hardcoded constants and use defined symbols for these
values.

Change-Id: I3edcf809572cb8c8360af19dcab7a12c4d1be4a9
Reviewed-on: http://gerrit.openafs.org/10528
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Tested-by: BuildBot <buildbot@rampaginggeek.com>