afs: Sanity-check some AFSFetchStatus structures

afs: Sanity-check some AFSFetchStatus structures

We currently do not do any sanity checking on the AFSFetchStatus
structures returned from fileservers. Add some sanity checking for
BulkStatus and FetchStatus calls, so we do not screw up our cache if a
fileserver gives us bogus data.

If we do get an invalid AFSFetchStatus structure, act as if the server
gave us a VBUSY error code, so we will retry the request. For OpenAFS
fileservers prior to 1.6.1 that yield this situation, VBUSY is likely
the error code the fileserver should have responded anyway.

Reviewed-on: http://gerrit.openafs.org/6880
Reviewed-by: Alistair Ferguson <alistair.ferguson@mac.com>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementix.org>
(cherry picked from commit 5af63fabc553a2fecd4c3080b25fe14483f5bd98)

Change-Id: I88922a75ada96d641bfd0078a8e7d6854c4ea699
Reviewed-on: http://gerrit.openafs.org/7992
Reviewed-by: Derrick Brashear <shadow@dementix.org>
Tested-by: BuildBot <buildbot@rampaginggeek.com>