net: openvswitch: fix possible memory leak in ovs_meter_cmd_set()

old_meter needs to be free after it is detached regardless of whether
the new meter is successfully attached.

Fixes: c7c4c44c ("net: openvswitch: expand the meters supported number")
Signed-off-by: Hangyu Hua <hbh25y@gmail.com>
Acked-by: Eelco Chaudron <echaudro@redhat.com>
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Signed-off-by: David S. Miller <davem@davemloft.net>