vlan: Fix register_vlan_dev() error path

In case register_netdevice() returns an error, and a new vlan_group
was allocated and inserted in vlan_group_hash[] we call
vlan_group_free() without deleting group from hash table. Future
lookups can give infinite loops or crashes.

We must delete the vlan_group using RCU safe procedure.

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>