vlan: add support for IFLA_VLAN_PROTOCOL

This patch adds support for IFLA_VLAN_PROTOCOL

Signed-off-by: Susant Sahani <susant@redhat.com>
Acked-by: Thomas Graf <tgraf@suug.ch>
[thaller@redhat.com: minor fixes (whitespace, documentation, and a typo)]
Signed-off-by: Thomas Haller <thaller@redhat.com>