Resolve error return issues in writepage

Resolve error return issues in writepage

The writepage_sync changes get error returns wrong in a couple of places. In
particular, they return a 0 code from dopartialwrite in preference to the
length return from page_writeback

Change-Id: I34a848fed5f799aa6844e9ef0339321f91c7e59b
Reviewed-on: http://gerrit.openafs.org/721
Reviewed-by: Marc Dionne <marc.c.dionne@gmail.com>
Tested-by: Simon Wilkinson <sxw@inf.ed.ac.uk>
Tested-by: Derrick Brashear <shadow@dementia.org>
Reviewed-by: Derrick Brashear <shadow@dementia.org>