net: route: Fix pkt leak if net_send_data() fails

If the call to net_send_data() fails, for example if the forwading
interface is down, then the pkt will leak. The reference taken by
net_pkt_shallow_clone() will never be released. Fix the problem
by dropping the rerefence count in the error path.

Signed-off-by: Andriy Gelman <andriy.gelman@gmail.com>
(cherry picked from commit a3cdb210)