os: io_event.res is the size written

And not an error code to be converted with cpp_strerror()

https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/tree/fs/aio.c#n1043

and 

http://tracker.ceph.com/issues/9570#note-19 shows how it can be used. It is not properly documented.

Signed-off-by: Loic Dachary <loic-201408@dachary.org>