datapath: Dump flow actions only if there is room.

datapath: Dump flow actions only if there is room.

Expanding an skbuff in a netlink dump handler doesn't work well.  We
weren't updating the truesize of the skb or the allocation within the
socket that netlink_dump() had put the skb in.  The code had other bugs
too.

This commit fixes the problem (in my tests, anyway) by avoiding expanding
the reply skbuff to fill in the actions.  Instead, in such a case the
userspace client has to do a separate "get" action to get the actions.
This commit also updates userspace to do this automatically for dumps in
the cases where the caller cares (only "ovs-dpctl dump-flows" currently
cares).

Signed-off-by: Ben Pfaff <blp@nicira.com>
Acked-by: Jesse Gross <jesse@nicira.com>
Bug #4520.