ofproto-dpif-xlate: Suppress oversize datapath actions.

If we allow oversize datapath actions to make it out of translation, then
we will assert-fail later when we try to put those actions into a Netlink
attribute.

Bug #19277.
Reported-by: Paul ingram <paul@nicira.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>