DAFS: Allow FSSYNC salvages on unknown volumes

DAFS: Allow FSSYNC salvages on unknown volumes

Allow salvage requests over FSSYNC (FORCE_ERROR with the FSYNC_SALVAGE
reason code) that are requested on volumes that we do not know to
exist. This can be helpful if a salvage is requested on a volume that
someone attempted to create but failed, indicating that a
partially-created volume may be in the way.

Log an additional message when we do this, as it should not be a
normal occurrence.

Change-Id: I6e1bbe35111a41c489fe85cad36e6a8a6f323a5a
Reviewed-on: http://gerrit.openafs.org/2285
Reviewed-by: Tom Keiser <tkeiser@sinenomine.net>
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>