selftests: forwarding: pedit_dsfield: Check counter value

A missing stats_update callback was recently added to act_pedit. Now that
iproute2 supports JSON dumping for pedit, extend the pedit_dsfield selftest
with a check that would have caught the fact that the callback was missing.

Signed-off-by: Petr Machata <petrm@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>