bridge: vni: Move open_json_object() within print_vni()

bridge: vni: Move open_json_object() within print_vni()

print_vni() is used to output one vni or vni range which, in json output
mode, looks like
      {
        "vni": 100
      }

Currently, the closing bracket is handled within the function but the
opening bracket is handled by open_json_object() before calling the
function. For consistency, move the call to open_json_object() within
print_vni().

Reviewed-by: Petr Machata <petrm@nvidia.com>
Tested-by: Petr Machata <petrm@nvidia.com>
Signed-off-by: Benjamin Poirier <bpoirier@nvidia.com>
Acked-by: Stephen Hemminger <stephen@networkplumber.org>
Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>