DAFS: Salvage VG on volume creation error

DAFS: Salvage VG on volume creation error

When trying to create a volume (either an entirely new volume or a
clone), request a demand-salvage on that volume group if we hit an
unexpected error. This can allow some situations to automatically
rectify themselves if, for example, a volume is missing its .vol
header, but still otherwise exists and causes an error during a clone.

Change-Id: I22b3e9028685395a8e621962138dee9f5f2ec822
Reviewed-on: http://gerrit.openafs.org/2286
Tested-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>