ofproto-dpif-xlate: Handle oversized actions more gracefully.

ofproto-dpif-xlate: Handle oversized actions more gracefully.

If the datapath actions exceed the maximum size of a Netlink attribute
(about 64 kB), then previously we would assert-fail (before commit
542024c4c3d36 "ofproto-dpif-xlate: Suppress oversize datapath actions.")
or just drop all of them (after that commit).  This commit makes OVS cope
by slow-pathing the flow and executing all of its actions in userspace.

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