net: dsa: xrs700x: Use irqsave variant for u64 stats update

xrs700x_read_port_counters() updates the stats from a worker using the
u64_stats_update_begin() version. This is okay on 32-UP since on the
reader side preemption is disabled.
On 32bit-SMP the writer can be preempted by the reader at which point
the reader will spin on the seqcount until writer continues and
completes the update.

Assigning the mib_mutex mutex to the underlying seqcount would ensure
proper synchronisation. The API for that on the u64_stats_init() side
isn't available. Since it is the only user, just use disable interrupts
during the update.

Use u64_stats_update_begin_irqsave() on the writer side to ensure an
uninterrupted update.

Fixes: ee00b24f32eb8 ("net: dsa: add Arrow SpeedChips XRS700x driver")
Cc: Andrew Lunn <[email protected]>
Cc: Florian Fainelli <[email protected]>
Cc: George McCollister <[email protected]>
Cc: Vivien Didelot <[email protected]>
Cc: Vladimir Oltean <[email protected]>
Signed-off-by: Sebastian Andrzej Siewior <[email protected]>
Acked-by: George McCollister <[email protected]>
Signed-off-by: David S. Miller <[email protected]>

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