salvager: Do not set fileName on header fixup

Currently, SalvageVolumeHeaderFile will set isp->volSummary->fileName
to a new string whenever the volume header needs to be created or
re-written. When control reaches back to SalvageFileSys1, this can
cause DeleteExtraVolumeHeaderFile to delete the header, since
vsp->fileName is used as a sort of indicator to see whether or not a
volume has been referenced by the inode summary.

When we create a new header, we avoid this because we allocate a new
VolumeSummary struct, which is not caught by the last
DeleteExtraVolumeHeaderFile for loop in SalvageFileSys1. However, we
do delete the header when we simply re-write a header, since we use
the existing VolumeSummary struct. Set fileName in neither, for
consistency.

Reviewed-on: http://gerrit.openafs.org/6783
Reviewed-by: Derrick Brashear <shadow@dementix.org>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
(cherry picked from commit 76f12c2389fd2a8e09b4e869730169401d154ce9)

Change-Id: I4d59de02a190a6d175735482aa102a496407f32a
Reviewed-on: http://gerrit.openafs.org/9457
Reviewed-by: Derrick 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>