[PKT_SCHED] act_gact: division by zero

Not returning -EINVAL, because someone might want to use the value
zero in some future gact_prob algorithm?

Signed-off-by: Kim Nordlund <kim.nordlund@nokia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Adrian Bunk <bunk@stusta.de>