files: make_gen_id -- Promote to be general helper

It is used in files tree generation so we will need
reuse for epoll sake.

Also use the whole 64 bit offset to shuffle bits more.

Signed-off-by: Cyrill Gorcunov <gorcunov@gmail.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>