test/xattr.c: remember to return cqe->res as the value

A recent commit broke this test, we rely on returning the actual read
or written value. Get that fixed up.

Fixes: a4f755f1f7e5 ("test/xattr.c: don't fail if the file system doesn't support xattr")
Signed-off-by: Jens Axboe <axboe@kernel.dk>