net: sched: mq: request stats from offloads

MQ doesn't hold any statistics on its own, however, statistic
from offloads are requested starting from the root, hence MQ
will read the old values for its sums.  Call into the drivers,
because of the additive nature of the stats drivers are aware
of how much "pending updates" they have to children of the MQ.
Since MQ reset its stats on every dump we can simply offset
the stats, predicting how stats of offloaded children will
change.

Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>