net: vxlan: convert to act as a pernet subsystem

As per suggestion from Eric W. Biederman, vxlan should be using
{un,}register_pernet_subsys() instead of {un,}register_pernet_device()
to ensure the vxlan_net structure is initialized before and cleaned
up after all network devices in a given network namespace i.e. when
dealing with network notifiers. This is similarly handeled already in
commit 91e2ff35 ("net: Teach vlans to cleanup as a pernet subsystem")
and, thus, improves upon fd27e0d4 ("net: vxlan: do not use vxlan_net
before checking event type"). Just as in 91e2ff35, we do not need
to explicitly handle deletion of vxlan devices as network namespace
exit calls dellink on all remaining virtual devices, and
rtnl_link_unregister() calls dellink on all outstanding devices in that
network namespace, so we can entirely drop the pernet exit operation
as well. Moreover, on vxlan module exit, rcu_barrier() is called by
netns since commit 3a765eda

 ("netns: Add an explicit rcu_barrier
to unregister_pernet_{device|subsys}"), so this may be omitted. Tested
with various scenarios and works well on my side.

Suggested-by: Eric W. Biederman <ebiederm@xmission.com>
Cc: Jesse Brandeburg <jesse.brandeburg@intel.com>
Cc: Eric W. Biederman <ebiederm@xmission.com>
Signed-off-by: Daniel Borkmann <dborkman@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>