revalidator: Eliminate duplicate flow handling.

A series of bugs have been identified recently that are caused by a
combination of the awkward flow dump API, possibility of duplicate flows
in a flow dump, and premature optimisation of the revalidator logic.
This patch attempts to simplify the revalidator logic by combining
multiple critical sections into one, which should make the state more
consistent.

The new flow of logic is:
+ Lookup the ukey.
+ If the ukey doesn't exist, create it.
+ Insert the ukey into the udpif. If we can't insert it, skip this flow.
+ Lock the ukey. If we can't lock it, skip it.
+ Determine if the ukey was already handled. If it has, skip it.
+ Revalidate.
+ Update ukey's fields (mark, flow_exists).
+ Unlock the ukey.

Previously, we would attempt process a flow without creating a ukey if
it hadn't been dumped before and it was due to be deleted. This patch
changes this to always create a ukey, allowing the ukey's
mutex to be used as the basis for preventing a flow from being handled
twice. This improves code correctness and readability.

Signed-off-by: Joe Stringer <joestringer@nicira.com>
Acked-by: Ethan Jackson <ethan@nicira.com>