fs/affs: bugfix: Write files greater than page size on OFS

Previous AFFS patch fixed OFS write operations but unveiled
another bug: files greater than 4KB are being created with a wrong
size resulting in errors like the following:

dd if=/dev/zero of=file bs=4097 count=1
cp file /mnt/affs/
cp: error writing '/mnt/affs/file': Bad address

Signed-off-by: Fabian Frederick <fabf@skynet.be>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>