dpif: Support fetching flow mask via dpif_flow_get().

Change the interface to allow implementations to pass back a buffer, and
allow callers to specify which of actions, mask, and stats they wish to
receive. This will be used in the next commit.

Signed-off-by: Joe Stringer <joestringer@nicira.com>
Acked-by: Ben Pfaff <blp@nicira.com>
revalidator: Revalidate missed flows.

If the datapath doesn't dump a flow for some reason, and the current
dump is expected to revalidate all flows in the datapath, then perform
revalidation for those flows by fetching them during the sweep phase.
If revalidation is not required, then leave the flow in the datapath and
don't revalidate it.

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