fix the regression from "direct-io: Fix negative return from dio read beyond eof"

Sure, it's better to bail out of past-the-eof read and return 0 than return
a bogus negative value on such.  Only we'd better make sure we are bailing out
with 0 and not -ENOMEM...

Cc: stable@vger.kernel.org
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>