[NETNS]: Inet control socket should not hold a namespace.

This is a generic requirement, so make inet_ctl_sock_create namespace
aware and create a inet_ctl_sock_destroy wrapper around
sk_release_kernel.

Signed-off-by: Denis V. Lunev <den@openvz.org>
Acked-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>