net: sched: act_ct: remove redundant variable err

Return value directly from pskb_trim_rcsum() instead of
getting value from redundant variable err.

Reported-by: Zeal Robot <zealci@zte.com.cn>
Signed-off-by: Jinpeng Cui <cui.jinpeng2@zte.com.cn>
Signed-off-by: David S. Miller <davem@davemloft.net>