net: sched: fix block->refcnt decrement

Currently the refcnt is never decremented in case the value is not 1.
Fix it by adding decrement in case the refcnt is not 1.

Reported-by: Vlad Buslov <vladbu@mellanox.com>
Fixes: f71e0ca4

 ("net: sched: Avoid implicit chain 0 creation")
Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>