auth: pass the directory name to _afsconf_CellServDBPath

auth: pass the directory name to _afsconf_CellServDBPath

Change the signature of the _afsconf_CellServDBPath() static function to
take just the base directory name of the CellServDB file instead of the
entire afsconf_dir data object. This makes it clear we do not need other
members of the afsconf_dir structure to compose the CellServDB path.

Change-Id: I57509b2ca09123e78df5533d63494c66b5b24cdf
Reviewed-on: https://gerrit.openafs.org/14076
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Andrew Deason <adeason@sinenomine.net>