bcrypt keys are on the rxkad list, not rxgk

Make the code match the comment.

Change-Id: If891b9a4b33eafc93816077c321790f5af10803f
Reviewed-on: http://gerrit.openafs.org/10582
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: D Brashear <shadow@your-file-system.com>