vxlan: make vxlan_sock_add and vxlan_sock_release complementary

Make vxlan_sock_add both alloc the socket and attach it to vxlan_dev. Let
vxlan_sock_release accept vxlan_dev as its parameter instead of vxlan_sock.

This makes vxlan_sock_add and vxlan_sock release complementary. It reduces
code duplication in the next patch.

Signed-off-by: Jiri Benc <jbenc@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>