DAFS: Deal with exclusive-state volume headers

DAFS: Deal with exclusive-state volume headers

GetVolumeHeader assumes that headers on the LRU are not associated
with a volume in an exclusive state. This is known to not be true for
some cases when salvage requests are received over FSSYNC, and may be
true in other scenarios. It's easy to just skip such headers, so skip
them.

Change-Id: I343d2e4f7af1c753641767be322420f38873c217
Reviewed-on: http://gerrit.openafs.org/5808
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementix.org>