net: dsa: Add support for 64-bit statistics

DSA slave network devices maintain a pair of bytes and packets counters
for each directions, but these are not 64-bit capable. Re-use
pcpu_sw_netstats which contains exactly what we need for that purpose
and update the code path to report 64-bit capable statistics.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>