rxkad: Consolidate refCount decrement

rxkad: Consolidate refCount decrement

To make code changes easier, all rxkad code that deals with
incrementing/decrementing the rx_securityClass refCount should be done
in the same place. Right now, there's only one incrementing code path,
but there are two decrement code paths that are duplicated. Change the
rxkad_DestroyConnection refCount-handling logic to just call
rxkad_Close(), since the behavior is identical.

Change-Id: Ibee79435dc62d79f0094ead818566c7f62c91898
Reviewed-on: https://gerrit.openafs.org/15153
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Cheyenne Wills <cwills@sinenomine.net>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>