Windows: cm_SetupStoreBIOD use firstModOffset chunk

Windows: cm_SetupStoreBIOD use firstModOffset chunk

When cm_SetupStoreBIOD attempts to store a chunk to the file
server it should not use *inOffsetp as the start of the range.
There is no guarantee that the buffer at *inOffsetp is dirty.
Instead use firstModOffset which refers to the first known
dirty buffer in the range specified by the caller.  Attempt
to fill a chunk of consecutive dirty buffers from that point.

Change-Id: I468e896b046192461d6fe0dc3669e5feeb7886ba
Reviewed-on: http://gerrit.openafs.org/6057
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Tested-by: Jeffrey Altman <jaltman@secure-endpoints.com>
Reviewed-by: Jeffrey Altman <jaltman@secure-endpoints.com>