afs: discard cached state when we are unsure of validity

afs: discard cached state when we are unsure of validity

in the event we got a network error, we don't know if the server
completed (or will complete) our operation. we can assume nothing.
a more complicated version of this could attempt to verify that the
state is what we expect it to be, but in extended callbacks universe
this is potentially easier to solve anyway. for now, return the
error to the caller, and mark the vcache unstat'd.

(cherry picked from commit c2fc7e0f66621fc97f5b4dc389d379260638315c)

Change-Id: Ic38cf16e47664e6f36ad614735b42d3f4e5a6ce2
Reviewed-on: http://gerrit.openafs.org/6520
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementix.org>