revalidator: Re-fix a flow duplication bug.

Commit 73a3c4757e59 (revalidator: Prevent handling the same flow twice.)
fixed a bug where duplicated flows could be deleted twice. Commit
7d1700980b5d (ofproto-dpif-upcall: Remove the flow_dumper thread.)
partially re-introduced this bug.

The bug would cause the logs to show messages such as
"failed to flow_get (No such file or directory) skb_priority(0),..."
"failed to flow_del (No such file or directory) skb_priority(0),..."

This patch fixes the issue again.

Signed-off-by: Joe Stringer <joestringer@nicira.com>
Acked-by: Alex Wang <alexw@nicira.com>