geneve: add rtnl changelink support

This patch adds changelink rtnl operation support for geneve devices
and the code changes involve:

  - added geneve_quiesce() which quiesces the geneve device data path
    for both TX and RX. This lets us perform the changelink operation
    atomically w.r.t data path. Also added geneve_unquiesce() to
    reverse the operation of geneve_quiesce().

  - refactor geneve_newlink into geneve_nl2info to be used by both
    geneve_newlink and geneve_changelink

  - geneve_nl2info takes a changelink boolean argument to isolate
    changelink checks.

  - Allow changing only a few attributes (ttl, tos, and remote tunnel
    endpoint IP address (within the same address family)):
    - 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: Girish Moodalbail <girish.moodalbail@oracle.com>
Acked-by: Pravin B Shelar <pshelar@ovn.org>
Signed-off-by: David S. Miller <davem@davemloft.net>