salvager: Handle multiple/inconsistent linktables

The ListAFSSubDirs code in namei_ops.c currently detects
incorrectly-named linktable files, and whines about them and says the
salvager will handle them. However, the salvager doesn't really handle
them, since we just use the first linktable we find (FindLinkHandle)
without checking any of the information about it.

So, check for these. Fix FindLinkHandle to only consider a linktable
the "real" linktable to use if it actually matches the volume group id
we're salvaging. Also delete any inconsistent linktables via the new
function CheckDupLinktable later on.

Note that inconsistently-named linktables have been known to have been
created in the past due to a bug in the salvager (fixed by ae227049),
and possibly due to other unknown issues.

Reviewed-on: http://gerrit.openafs.org/10322
Reviewed-by: Mark Vitale <mvitale@sinenomine.net>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: D Brashear <shadow@your-file-system.com>
(cherry picked from commit 602e8eb2000be02ef2a6627633b7ba80ea847762)

Change-Id: I472e250bbe5dcb4de44111ac705c9a319abf2b44
Reviewed-on: http://gerrit.openafs.org/10811
Reviewed-by: D Brashear <shadow@your-file-system.com>
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>