flow_offload: fix block stats

With the introduction of flow_stats_update(), drivers now update the stats
fields of the passed tc_cls_flower_offload struct, rather than call
tcf_exts_stats_update() directly to update the stats of offloaded TC
flower rules. However, if multiple qdiscs are registered to a TC shared
block and a flower rule is applied, then, when getting stats for the rule,
multiple callbacks may be made.

Take this into consideration by modifying flow_stats_update to gather the
stats from all callbacks. Currently, the values in tc_cls_flower_offload
only account for the last stats callback in the list.

Fixes: 3b1903ef

 ("flow_offload: add statistics retrieval infrastructure and use it")
Signed-off-by: John Hurley <john.hurley@netronome.com>
Reviewed-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Acked-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: David S. Miller <davem@davemloft.net>