openvswitch: Minimize ovs_flow_cmd_del critical section.

ovs_flow_cmd_del() now allocates reply (if needed) after the flow has
already been removed from the flow table.  If the reply allocation
fails, a netlink error is signaled with netlink_set_err(), as is
already done in ovs_flow_cmd_new_or_set() in the similar situation.

Signed-off-by: Jarno Rajahalme <jrajahalme@nicira.com>
Signed-off-by: Pravin B Shelar <pshelar@nicira.com>