sockets, inet: Use general machnism for checkpoint/restore v2

Use fdtype_ops facility to c/r inet sockets.

v2:
 - Use BUG_ON if socket is attempted to be dumped
   several times

Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>