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.

Change-Id: I67e0205362d4c08ea9a8c12104fea8bfe8cbf798
Reviewed-on: http://gerrit.openafs.org/2231
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>