ixv: Preserve statistics across resetsHEADmain

The VF statistics registers are free running and are not cleared on
read.  The existing code records attach time bases and pre-reset totals,
but never uses either when publishing counters.  It instead replaces
the low hardware bits directly, so counters can inherit pre-attach
traffic or jump backward after a reset.

Accumulate modular 32- and 36-bit deltas, following DPDK, while keeping
the software totals across planned resets.  Establish a fresh hardware
baseline after each successful reset and invalidate the sampling epoch
when mailbox state is lost.  Detect unsolicited PF resets explicitly so
a reset while link is down cannot be mistaken for counter wrap.

Remove the unused base and saved-reset bookkeeping.

On E610, packet and octet counters remained monotonic across a VF FLR
and a PF down/up cycle.  Traffic after each reset advanced both RX and
TX counters.

MFC after:	2 weeks
Sponsored by:	BBOX.io

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