vol: avoid attach retry loops on SYNC_FAILED

vol: avoid attach retry loops on SYNC_FAILED

Avoid unnecessary volume attach retry loops if a volume
salvage is unable to be scheduled due to a hard error.

Change-Id: If1962b83e03f634044a55b03787661b8a7695008
Reviewed-on: http://gerrit.openafs.org/7566
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Tom Keiser <tkeiser@sinenomine.net>
Reviewed-by: Derrick Brashear <shadow@dementix.org>