Unix CM: Avoid using stale DV in afs_StoreAllSegments

It was reported in RT 131976 that on Linux some file
corruption was observed when doing mmap writes to
a file substantially larger than the cache size.

osi_VM_StoreAllSegments drops locks and asks the OS to flush
any dirty pages in the file 's mapping.  This will trigger
calls into our writepage op, and if the number of dirty
cache chunks is too high (as will happen for a file larger
than the cache size), afs_DoPartialWrite will recursively
call afs_StoreAllSegments and some chunks will be written
back to the server.  After potentially doing this several
times, control will return to the original afs_StoreAllSegments.

At that point the data version that was stored before
osi_VM_StoreAllSegments is no longer correct, leading to
possible data corruption.

Triggering this bug requires writing a file larger than the
cache so that partial stores are done, and writing enough
data to exceed the system's maximum dirty ratio and cause
it to initiate writeback.

To fix, just wait until after osi_VM_StoreAllSegments to
look at and store the data version.

FIXES 131976

Change-Id: I959f06b5a7a51171e7ed70189620e9d11d52efa2
Reviewed-on: http://gerrit.openafs.org/11644
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>