Make suterusu work on recent kernels

f_vfsmnt was removed in 3.9 kernel (and was #defined as f_path.mnt in older kernels)
http://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=182be684784334598eee1d90274e7f7aa0063616

Starting from 3.11, iterate() is used instead of readdir(), see http://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=2233f31aade393641f0eaed43a71110e629bb900
I'm not terribly happy with preprocessor stuff, but can't think of anything better. Also, for consistency I remove o_ prefix from filldir and readdir functions.
Tested on 3.11.0-12 2.6.38-8