net: dsa: felix: check the 32-bit PSFP stats against overflow

The Felix PSFP counters suffer from the same problem as the ocelot
ndo_get_stats64 ones - they are 32-bit, so they can easily overflow and
this can easily go undetected.

Add a custom hook in ocelot_check_stats_work() through which driver
specific actions can be taken, and update the stats for the existing
PSFP filters from that hook.

Previously, vsc9959_psfp_filter_add() and vsc9959_psfp_filter_del() were
serialized with respect to each other via rtnl_lock(). However, with the
new entry point into &psfp->sfi_list coming from the periodic worker, we
now need an explicit mutex to serialize access to these lists.

We used to keep a struct felix_stream_filter_counters on stack, through
which vsc9959_psfp_stats_get() - a FLOW_CLS_STATS callback - would
retrieve data from vsc9959_psfp_counters_get(). We need to become
smarter about that in 3 ways:

- we need to keep a persistent set of counters for each stream instead
  of keeping them on stack

- we need to promote those counters from u32 to u64, and create a
  procedure that properly keeps 64-bit counters. Since we clear the
  hardware counters anyway, and we poll every 2 seconds, a simple
  increment of a u64 counter with a u32 value will perfectly do the job.

- FLOW_CLS_STATS also expect incremental counters, so we also need to
  zeroize our u64 counters every time sch_flower calls us

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>