cls_flower: Fix incorrect idr release when failing to modify rule

When we fail to modify a rule, we incorrectly release the idr handle
of the unmodified old rule.

Fix that by checking if we need to release it.

Fixes: fe2502e4 ("net_sched: remove cls_flower idr on failure")
Reported-by: Vlad Buslov <vladbu@mellanox.com>
Reviewed-by: Roi Dayan <roid@mellanox.com>
Acked-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: Paul Blakey <paulb@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>