DAFS: Remove VOL_SALVAGE_INVALIDATE_HEADER

DAFS: Remove VOL_SALVAGE_INVALIDATE_HEADER

Currently VRequestSalvage_r takes a flag,
VOL_SALVAGE_INVALIDATE_HEADER, which causes the header for the
specified volume to be freed (via FreeVolumeHeader). This is almost
never safe to do, since there may be other users of the specified
volume that can be accessing the volume header at the same time.

There is also no reason to invalidate the header at the time of the
VRequestSalvage_r call, since the header must be invalidated when we
detach the volume (other utilities may change header information). So,
if there are any problems in the future because we do not invalidate
the header at the time of VRequestSalvage_r, it is the fault of the
detachment/offlining logic.

So, remove VOL_SALVAGE_INVALIDATE_HEADER and all of its users. Take
this opportunity to correctly document the VRequestSalvage_r headers
in the VRequestSalvage_r comment, as it was previously missing the
VOL_SALVAGE_NO_OFFLINE flag.

Reviewed-on: http://gerrit.openafs.org/5319
Reviewed-by: Derrick Brashear <shadow@dementix.org>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
(cherry picked from commit 4552dc552687267fce3c7a6a9c7f4a1e9395c8e5)

Change-Id: Ib991ecefea2115a3c4308ed3c261af7433c9b858
Reviewed-on: http://gerrit.openafs.org/5722
Reviewed-by: Derrick Brashear <shadow@dementix.org>
Tested-by: Derrick Brashear <shadow@dementix.org>