Almost all read/write operation handles data with chunks(segments or pages)
and result has integral behaviour for folowing scenario: 
for_each_chunk() {
     res = op(....);
     if(IS_ERROR(res))
           return progress ? progress : res;
     progress += res;
}
prepare_write may has integral behaviour in case of blksize < pgsize,
for example we successfully allocated/read some blocks, but not all of them,
and than some error happend. Currently we eliminate this progress by doing
vmtrunate() after prepare_has failed.
It is good to have ability to signal about this progress. Interprete positive
prepare_write() ret code as bytes num that fs ready to handle at this moment.
I've ask SAW, he think it is sane. This size always less than PAGE_CACHE_SIZE
so it less than AOP_TRUNCATED_PAGE too.
 
BTH: This approach was used in OpenVZ 2.6.9 kernel in order to make FS with 
delayed allocation more correct, and its works well.
I think not everybody will happy about this,  but let's discuss all advantages
and disadvantages of this change.

Signed-off-by: Dmitriy Monakhov <dmonakhov@openvz.org>
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