net/mlx5e: Expose rx_oversize_pkts_buffer counter

Add the rx_oversize_pkts_buffer counter to ethtool statistics.
This counter exposes the number of dropped received packets due to
length which arrived to RQ and exceed software buffer size allocated by
the device for incoming traffic. It might imply that the device MTU is
larger than the software buffers size.

Signed-off-by: Gal Pressman <gal@nvidia.com>
Reviewed-by: Tariq Toukan <tariqt@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>