sfc: add n_rx_overlength to ethtool stats

The previous patch changed when we increment the RX queue's rx_packets
 counter, to match the semantics of netdev per-queue stats.  The
 differences between the old and new counts are scatter errors (which
 produce a WARN_ON) and this counter, which is incremented by
 efx_rx_packet__check_len() when an RX packet (which was placed in a
 single buffer by SG, i.e. n_frags == 1) has a length (from the RX
 event) which is too long to fit in the RX buffer.  If this occurs, we
 drop the packet and fire a ratelimited netif_err().
The counter previously was not reported anywhere; add it to ethtool -S
 output to ensure users still have this information.

Reviewed-by: Jacob Keller <jacob.e.keller@intel.com>
Signed-off-by: Edward Cree <ecree.xilinx@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>