ubik: Fix buffers for reading-during-writes

ubik: Fix buffers for reading-during-writes

If we are reading while a write transaction is in progress, we can
encounter a buffer that is dirty if we're on the same site as the
writer. Ignore these buffers for readers, since they contain
uncommitted changes. Then, when the writer commits, invalidate the
resultant duplicate buffer, if one exists.

Reviewed-on: http://gerrit.openafs.org/2231
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>
(cherry picked from commit 570236dd55e79e8886b45f19cef59ec3fa9d53f6)

Change-Id: If83e077b99a77339377a36f1d1bac9182ef26e4f
Reviewed-on: http://gerrit.openafs.org/6260
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementix.org>