ethtool: add interface to read RMON stats

ethtool: add interface to read RMON stats

Most devices maintain RMON (RFC 2819) stats - particularly
the "histogram" of packets received by size. Unlike other
RFCs which duplicate IEEE stats, the short/oversized frame
counters in RMON don't seem to match IEEE stats 1-to-1 either,
so expose those, too. Do not expose basic packet, CRC errors
etc - those are already otherwise covered.

Because standard defines packet ranges only up to 1518, and
everything above that should theoretically be "oversized"
- devices often create their own ranges.

Going beyond what the RFC defines - expose the "histogram"
in the Tx direction (assume for now that the ranges will
be the same).

Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>