netns: Fix icmp shutdown.

Recently I had a kernel panic in icmp_send during a network namespace
cleanup.  There were packets in the arp queue that failed to be sent
and we attempted to generate an ICMP host unreachable message, but
failed because icmp_sk_exit had already been called.

The network devices are removed from a network namespace and their
arp queues are flushed before we do attempt to shutdown subsystems
so this error should have been impossible.

It turns out icmp_init is using register_pernet_device instead
of register_pernet_subsys.  Which resulted in icmp being shut down
while we still had the possibility of packets in flight, making
a nasty NULL pointer deference in interrupt context possible.

Changing this to register_pernet_subsys fixes the problem in
my testing.

Signed-off-by: Eric W. Biederman <ebiederm@aristanetworks.com>
Acked-by: Denis V. Lunev <den@openvz.org>
Signed-off-by: David S. Miller <davem@davemloft.net>