os: io_event.res is the size written

And not an error code to be converted with cpp_strerror()

Signed-off-by: Loic Dachary <loic-201408@dachary.org>
Merge pull request #2577 from dachary/wip-9570-cpp-strerror

os: io_event.res is the size written

Reviewed-By: Christophe Courtaut <christophe.courtaut@gmail.com>