vxlan: add changelink support

This patch adds changelink rtnl op support for vxlan netdevs.
code changes involve:
    - refactor vxlan_newlink into vxlan_nl2conf to be
    used by vxlan_newlink and vxlan_changelink
    - vxlan_nl2conf and vxlan_dev_configure take a
    changelink argument to isolate changelink checks
    and updates.
    - Allow changing only a few attributes:
        - return -EOPNOTSUPP for attributes that cannot
        be changed for now. Incremental patches can
        make the non-supported one available in the future
        if needed.
Signed-off-by: Roopa Prabhu <roopa@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>