dpif: When executing actions needs help, use "set" action to set tunnel.

Open vSwitch userspace is able to implement some actions that the kernel
doesn't support, such as modifying ARP fields.  When it does this for a
tunneled packet, it needs to supply the tunnel information with a "set"
action, because the Linux kernel datapath throws away tunnel information
supplied in the OVS_PACKET_CMD_EXECUTE metadata argument.

VMware-BZ: #1270110
Reported-by: Srinivas Neginhal <sneginha@vmware.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>
Acked-by: Jesse Gross <jesse@nicira.com>