DAFS: fix forceDAFS support in salvager

DAFS: fix forceDAFS support in salvager

we need to get the shared partition lock for the per-volume
forceDAFS case

Reviewed-on: http://gerrit.openafs.org/3487
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementia.org>
(cherry picked from commit 81e7a5adf93a6b5eec104ca4bde429cf6cb0548a)

Change-Id: Ie7c6a43d25c621a3faca2a6564fbc06f9af87880
Reviewed-on: http://gerrit.openafs.org/4278
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementia.org>