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.

Change-Id: Iafb67f24b89d78b8236660d047da12fce1dd6061
Reviewed-on: http://gerrit.openafs.org/6510
Reviewed-by: Derrick Brashear <shadow@dementix.org>
Tested-by: Derrick Brashear <shadow@dementix.org>