security: Remove rtnl_lock() in selinux_xfrm_notify_policyload()

rt_genid_bump_all() consists of ipv4 and ipv6 part.
ipv4 part is incrementing of net::ipv4::rt_genid,
and I see many places, where it's read without rtnl_lock().

ipv6 part calls __fib6_clean_all(), and it's also
called without rtnl_lock() in other places.

So, rtnl_lock() here was used to iterate net_namespace_list only,
and we can remove it.

Signed-off-by: Kirill Tkhai <ktkhai@virtuozzo.com>
Signed-off-by: David S. Miller <davem@davemloft.net>