selftests: pmtu: Use namespace command prefix to fetch route mtu

In 7af137b72131 ("selftests: net: Introduce first PMTU test") I
accidentally assumed route_get_* helpers would run from a single
namespace. Make them a bit more generic, by passing the
namespace command prefix as a parameter instead.

Fixes: 7af137b72131 ("selftests: net: Introduce first PMTU test")
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>