geneve: support for modifying geneve device

geneve: support for modifying geneve device

Ability to change geneve device attributes was added to kernel through
commit 5b861f6baa3a ("geneve: add rtnl changelink support"), however one
cannot do the same through ip-link(8) command.  Changing the allowed
geneve device attributes using 'ip link set <geneve_name> type geneve id
<geneve_id> <allowed_attributes>' currently fails with 'operation not
supported' error.  This patch adds support for it.

Signed-off-by: Girish Moodalbail <girish.moodalbail@oracle.com>