Windows: RequestExtents avoid bufWrite if rdr held

Windows: RequestExtents avoid bufWrite if rdr held

If the cm_buf_t is held by the redirector the buffer cannot
be written back to the file server even if dirty.  Therefore,
do not check whether or not the cm_buf_t is dirty until after
it is known that the buffer is not redirector owned.

Change-Id: I10dc8f74915c2267dc44138284eba273eb708e0a
Reviewed-on: http://gerrit.openafs.org/6432
Reviewed-by: Jeffrey Altman <jaltman@secure-endpoints.com>
Tested-by: Jeffrey Altman <jaltman@secure-endpoints.com>