selftests: forwarding: lib: Add link_stats_rx_errors_get()

Such a function will be useful for counting malformed packets in the ECN
decap test.

To that end, introduce a common handler for handling stat-fetching, and
reuse it in link_stats_tx_packets_get() and link_stats_rx_errors_get().

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