afs: Check dv against localhero aincr

For operations that modify directories, we call afs_LocalHero to
determine if we can perform the directory modification in our local
cache, and avoid fetching the dir blob from the fileserver. Currently,
afs_LocalHero assumes that the DV received from the fileserver is
correct, and will update the cache DV as long as we have a valid
callback on the file.

If for any reason the client cache falls out of sync with what's on
the fileserver, this can cause the client to incorrectly believe its
cache is up to date. Since, the cached data will be marked with the
newest DV, even if the DV on the server has jumped to be larger than
we expected.

While the client cache should never fall out of sync with the
fileserver, in the past this has been possible due to other bugs
(fileserver idle dead processing and client VNOSERVICE handling).
Assuming that the given DV is correct is also just unnecesarily
fragile, since we can always check if it is correct, so just check it,
and add some comments helping explain what's going on here. Note that
regular file writes effectively already check this.

Note that this change makes use of the 'aincr' argument to
afs_LocalHero, which was previously unused. aincr appears to have been
used for a purpose similar to this before OpenAFS 1.0, but was
removed, possibly accidentally.

It is possible this change negatively affects, or even breaks
(unlikely), functionality with the AFS<->DFS translator. Although
nothing of the sort has been seen, it is difficult to know one way or
the other, due to the lack of available DFS translators.

Change-Id: I0e5395bac695257f66ba0cd58695a59ebdf56431
Reviewed-on: http://gerrit.openafs.org/8864
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>