fileserver: Fix NeverAttach support

Commit 35becabed870d4bfe49abaa499d99a3ffb0a2d31 added support for
the /vicepXX/NeverAttach. However this code only appears to work on
Linux. It fails build testing on (at least) Mac OS X, FreeBSD, and AIX.
Modify the code so that the NeverAttach call uses the same variable to
locate the path of the partition as the AlwaysAttach call does.

Reviewed-on: http://gerrit.openafs.org/7125
Reviewed-by: Jeffrey Altman <jaltman@secure-endpoints.com>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementix.org>
(cherry picked from commit 709a6358e1d1f6cc0b025522e1e595c50bb2b733)

Change-Id: I2f142084fdb4e66c3e7841c9ac961cab417633a0
Reviewed-on: http://gerrit.openafs.org/9471
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>
Tested-by: Stephan Wiesand <stephan.wiesand@desy.de>