vlan: introduce vlan_dev_free_egress_priority

This patch is to introduce vlan_dev_free_egress_priority() to
free egress priority for vlan dev, and keep vlan_dev_uninit()
static as .ndo_uninit. It makes the code more clear and safer
when adding new code in vlan_dev_uninit() in the future.

Signed-off-by: Xin Long <lucien.xin@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>