net: mscc: ocelot: add TX_MM_HOLD to ocelot_mm_stats_layout

The lack of a definition for this counter is what initially prompted me
to investigate a problem which really manifested itself as the previous
change, "net: mscc: ocelot: fix transfer from region->buf to ocelot->stats".

When TX_MM_HOLD is defined in enum ocelot_stat but not in struct
ocelot_stat_layout ocelot_mm_stats_layout, this creates a hole, which
due to the aforementioned bug, makes all counters following TX_MM_HOLD
be recorded off by one compared to their correct position. So for
example, a non-zero TX_PMAC_OCTETS would be reported as TX_MERGE_FRAGMENTS,
TX_PMAC_UNICAST would be reported as TX_PMAC_OCTETS, TX_PMAC_64 would be
reported as TX_PMAC_PAUSE, etc etc. This is because the size of the hole
(1) is much smaller than the size of the region, so the phenomenon where
the stats are off-by-one, rather than lost, prevails.

However, the phenomenon where stats are lost can be seen too, for
example with DROP_LOCAL, which is at the beginning of its own region
(offset 0x000400 vs the previous 0x0002b0 constitutes a discontinuity).
This is also reported as off by one and saved to TX_PMAC_1527_MAX, but
that counter is not reported to the unstructured "ethtool -S", as
opposed to DROP_LOCAL which is (as "drop_local").

Fixes: ab3f97a961 ("net: mscc: ocelot: export ethtool MAC Merge stats for Felix VSC9959")
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

drivers/net/ethernet/mscc/ocelot_stats.c

@@ -274,6 +274,7 @@ static const struct ocelot_stat_layout ocelot_mm_stats_layout[OCELOT_NUM_STATS]
 	OCELOT_STAT(RX_ASSEMBLY_OK),
 	OCELOT_STAT(RX_MERGE_FRAGMENTS),
 	OCELOT_STAT(TX_MERGE_FRAGMENTS),
+	OCELOT_STAT(TX_MM_HOLD),
 	OCELOT_STAT(RX_PMAC_OCTETS),
 	OCELOT_STAT(RX_PMAC_UNICAST),
 	OCELOT_STAT(RX_PMAC_MULTICAST),