net/sched: act_vlan: Introduce TCA_VLAN_ACT_MODIFY vlan action

TCA_VLAN_ACT_MODIFY allows one to change an existing tag.

It accepts same attributes as TCA_VLAN_ACT_PUSH (protocol, id,
priority).
If packet is vlan tagged, then the tag gets overwritten according to
user specified attributes.

For example, this allows user to replace a tag's vid while preserving
its priority bits (as opposed to "action vlan pop pipe action vlan push").

Signed-off-by: Shmulik Ladkani <shmulik.ladkani@gmail.com>
Acked-by: Jamal Hadi Salim <jhs@mojatatu.com>
Signed-off-by: David S. Miller <davem@davemloft.net>