selftests: net: vxlan: Fix selftest regression after changes in iproute2.

The iproute2 output that eventually landed upstream is different than
the one used in this test, resulting in failures. Fix by adjusting the
test to use iproute2's JSON output, which is more stable than regular
output.

Fixes: 305c0418 ("selftests: net: vxlan: Add tests for vxlan nolocalbypass option.")
Signed-off-by: Vladimir Nikishkin <vladimir@nikishkin.pw>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Tested-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>