revalidator: Don't delete non-existent flow.

If ukey_acquire() returns ENOENT, then it is unable to locate the ukey
corresponding to the flow and the flow has disappeared since it was
dumped. Don't bother deleting the flow in this case, as it will fail.

Signed-off-by: Joe Stringer <joestringer@nicira.com>
Acked-by: Thomas Graf <tgraf@noironetworks.com>