net/sched: act_ct: Fix byte count on fragmented packets

First fragmented packets (frag offset = 0) byte len is zeroed
when stolen by ip_defrag(). And since act_ct update the stats
only afterwards (at end of execute), bytes aren't correctly
accounted for such packets.

To fix this, move stats update to start of action execute.

Fixes: b57dc7c1

 ("net/sched: Introduce action ct")
Signed-off-by: Paul Blakey <paulb@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>