files: Merge collect_gen_fd() and collect_used_fd(), and call it unconditionally

Since we keep files of all types in a single list (fds), it's possible
to use only function for that and to call it unconditionally.

v4: New

Signed-off-by: Kirill Tkhai <ktkhai@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>