Changes from ver(1)
 - __page_symlink(): In order to be on a safe side add explicit zeroing content
                     before fail (just in case).
 -do_lo_send_aops(): If prepare_write can't handle total size of bytes requested
                     from loop_dev it is safer to fail.
 -  pipe_to_file():  Limit the size of the copy to size wich prepare_write ready
                     to handle. instead of failing.
 - ecryptfs: was't changed in this version, because where is no even 
                     AOP_TRUNCATED_PAGE handling code where.

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.
 
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.

fixes not dirrectly connected with proposed prepare_write semantic changes:
 __page_symlink : If find_or_create_page has failed on second retry attempt
                  function will exit with wrong error code.
 __generic_cont_expand: Add correct AOP_TRUNCATED_PAGE handling.

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