use proc_set_size instead of proc_dir_entry->size

for kernels >= 3.10.0

struct proc_dir_entry is no more public, so we have to use the new
interface methods.
use proc_mkdir instead of create_proc_entry for dirs

it exists at least since 2.6.26 (since around 2.4 even) and its default
modes work for us, too.
removed S_IWUGO from veth/vnet device attributes

otherwise we get a big fat WARN() since Linux 3.9.
theyre not "->store"ing, so this attribute is not needed.

read:
commit 8f46baaa7ec6cd0851794020b31958e64679dd26