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.

Reviewed-on: http://gerrit.openafs.org/5808
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementix.org>
(cherry picked from commit c2ee7b00ddfe8b1b8c296a99f0244eb33c726e8e)

Change-Id: I0eda511071114b01cd8984d30858b0aa9157727a
Reviewed-on: http://gerrit.openafs.org/6085
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementix.org>