upcall: Defer ukey deletion until after pushing stats.

upcall: Defer ukey deletion until after pushing stats.

It is possible for a datapath to dump the same flow twice, for instance
if the flow is the last in a batch of flows to be dumped, then a new
flow is inserted into the same bucket before the flow dumper fetches
another batch.

In this case, datapath flow stats may be duplicated: The revalidator
records the stats from the first flow, using the ukey to get the stats
delta. The ukey is deleted, then the revalidator reads the second
(duplicate) flow and cannot lookup the ukey for the delta. As such, it
will push the stats as-is.

This patch reduces the likelihood of such stats duplications by
deferring ukey deletion until after stats are pushed for deleted flows.

Signed-off-by: Joe Stringer <joestringer@nicira.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>