datapath: Change listing flows to use an iterator concept.

datapath: Change listing flows to use an iterator concept.

One of the goals for Open vSwitch is to decouple kernel and userspace
software, so that either one can be upgraded or rolled back independent of
the other.  To do this in full generality, it must be possible to change
the kernel's idea of the flow key separately from the userspace version.
In turn, that means that flow keys must become variable-length.  This does
not, however, fit in well with the ODP_FLOW_LIST ioctl in its current form,
because that would require userspace to know how much space to allocate
for each flow's key in advance, or to allocate as much space as could
possibly be needed.  Neither choice is very attractive.

This commit prepares for a different solution, by replacing ODP_FLOW_LIST
by a new ioctl ODP_FLOW_DUMP that retrieves a single flow from the datapath
on each call.  It is much cleaner to allocate the maximum amount of space
for a single flow key than to do so for possibly a very large number of
flow keys.

As a side effect, this patch also fixes a race condition that sometimes
made "ovs-dpctl dump-flows" print an error: previously, flows were listed
and then their actions were retrieved, which left a window in which
ovs-vswitchd could delete the flow.  Now dumping a flow and its actions is
a single step, closing that window.

Dumping all of the flows in a datapath is no longer an atomic step, so now
it is possible to miss some flows or see a single flow twice during
iteration, if the flow table is modified by another process.  It doesn't
look like this should be a problem for ovs-vswitchd.

It would be faster to retrieve a number of flows in batch instead of just
one at a time, but that will naturally happen later when the kernel
datapath interface is changed to use Netlink, so this patch does not bother
with it.

Signed-off-by: Ben Pfaff <blp@nicira.com>
Acked-by: Jesse Gross <jesse@nicira.com>