Alexey Dobriyan found, that virtualized netlink kernel sockets (fibl &
rtnl) are leaked during namespace start/stop loop.

Leaking fix (simple and obvious) reveals that netlink kernel socket
disposal leads to OOPSes:
- nl_table[protocol]->listeners is double freed
- sometimes during namespace stop netlink_sock_destruct
  BUG_TRAP(!atomic_read(&sk->sk_rmem_alloc)); is hit

This set address all these issues.

Signed-off-by: Denis V. Lunev <den@openvz.org>
Tested-by: Alexey Dobriayn <adobriyan@openvz.org>