Here's a patch to handle error return values in fs_flush_pages and
fs_flushinval_pages.  It changes the prototype of fs_flushinval_pages
so we can propogate the errors and handle them at higher layers.  I also 
modified xfs_itruncate_start so that it could propogate the error further.

I've changed the necessary prototypes on 2.4 to keep the build happy but
haven't bothered to fix the error handling in fs_flush_pages or 
fs_flushinval_pages for 2.4.

The motivation behind this change was the recent BUG reported due to a
direct I/O read trying to write to delayed alloc extents.  While the exact
cause of this problem is not known it is possible that fs_flushinval_pages
ignored an error while flushing, truncated the pages on the file anyway,
and failed to convert all delayed alloc extents.

Lachlan