qlnxe: Advertise the IFCAP_HWSTATS capability

[zlei]

Summary

The hardware can count statistics and the driver has already retrieved
them via qlnx_get_counter().

Advertise the IFCAP_HWSTATS capability so that the net stack will not
double count IFCOUNTER_IBYTES.

Note that on retrieving IFCOUNTER_IBYTES the count statistics stored
in netif->if_counters[IFCOUNTER_IBYTES] is ignored so this change will
not fix anything but is only a micro optimization.

MFC after: 1 week

[kbowling]

In the commit message do you mean to say there is something else double counting?

[zlei]

In D51451#1174756, @kbowling wrote:

In the commit message do you mean to say there is something else double counting?

I may not express that clearly.

Advertise the IFCAP_HWSTATS capability so that the net stack will not
double count IFCOUNTER_IBYTES.

See the logic in if_ethersubr.c https://cgit.freebsd.org/src/tree/sys/net/if_ethersubr.c#n587

So the counter ifp->if_counters[IFCOUNTER_IBYTES] actually increases, but qlnx_get_counter() just ignores it. https://cgit.freebsd.org/src/tree/sys/dev/qlnx/qlnxe/qlnx_os.c#n7164

[kbowling]

I would remove the phrase 'change will not fix anything but' in the commit message and I think this is good.

[zlei]

In D51451#1174919, @kbowling wrote:

I would remove the phrase 'change will not fix anything but' in the commit message and I think this is good.

OK, I'll revise before committing.