Fix prepare and commit_write to do the right thing

Fix prepare and commit_write to do the right thing

Even when we're doing syncronous writeback, as we currently do
for write() operations, it's important to correctly fill, and flag
the pages we're writing to. Not doing so has a huge performance
penalty, as it means even when we've just written a page, we have to
pull it back from the backing store for a read.

This code fixes prepare_write and commit_write (for RHEL5) and
write_begin and write_end (for Fedora) to correctly populate and
flag pages which are being written.

Change-Id: Iaa2165b9b429000dcf0c6dd452e3eb8033257277
Reviewed-on: http://gerrit.openafs.org/820
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>