bos: Do not assume DAFS just if DAFS bnode exists

bos: Do not assume DAFS just if DAFS bnode exists

For salvaging purposes, we were assuming that if a DAFS bnode existed,
we should perform a DAFS salvage. Since it is possible to have a
disabled DAFS bnode alongside an enabled non-DAFS fs bnode, we should
perform a regular non-DAFS salvage if the DAFS bnode is disabled. Do
that.

Change-Id: If4e6fa3b733e2e9684cd9942be56368d60bf6dee
Reviewed-on: http://gerrit.openafs.org/2453
Tested-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Derrick Brashear <shadow@dementia.org>