dpif: Don't synchronize flow_dump_next() status.

dpif: Don't synchronize flow_dump_next() status.

Recent changes to the flow_dump_next() interface have made it the
responsibility of the dpif implementation to track error status over a
flow dump operation.

This patch removes status tracking from 'struct dpif_flow_dump', allowing
multiple threads to call dpif_flow_dump_next() and track their status
independently. Even if one thread finishes processing flows for a given
iterator and state, it will not prevent other callers from processing
the remaining flows in their buffers.

After this patch, the error code that dpif_flow_dump_next() returns is
only significant for the current state and buffer. As before, the status
of the entire flow dump operation can be obtained by calling
dpif_flow_dump_done().

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