afs: Set DWriting when truncating a dcache entry

afs: Set DWriting when truncating a dcache entry

When we truncate a file, we truncate the contents of the relevant
dcache entry chunks, and prevent future FetchData operations from
fetching data beyond the truncation offset. If we never write anything
to that chunk, we never set DWriting, and so on disk it looks like
that dcache entry has valid data for the specified DV. However, since
the data is truncated, this is not true.

If a process holds a file open, truncates it without writing to it,
and then the client crashes (or we have trouble contacting the
fileserver when we close the file), the dcache entry will appear valid
on disk. So the next time we read the dcache entry, we will use the
incorrect cache contents as if they were accurate for the specified
DV.

To avoid this, set DWriting when we truncate a chunk. Normally we only
clear DWriting when we actually send data to the fileserver, so to
clear DWriting in this case, add an additional line to clear it in
afs_StoreAllSegments, after the StoreMini has completed.

Reviewed-on: http://gerrit.openafs.org/6937
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementix.org>
(cherry picked from commit b3b267d16eefb993f52f66bf44f3e659c1017e58)

Change-Id: I5602db9687f3f005f68200a4208a3c7012933efa
Reviewed-on: http://gerrit.openafs.org/7996
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementix.org>