vxlan: unregister on namespace exit

Fix memory leaks and other badness from VXLAN network namespace
teardown. When network namespace is removed, all the vxlan devices should
be unregistered (not closed).

Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>
Reviewed-by: Pravin B Shelar <pshelar@nicira.com>
Signed-off-by: David S. Miller <davem@davemloft.net>