tun-metadata: Always set option present when copying data.

tun-metadata: Always set option present when copying data.

Whenever we write into a tunnel option field, we also need to mark
it as significant. If we don't, then the data will later be ignored.

We currently do this in every case except for flow metadata. This causes
us to not correctly serialize the tunnel metadata for Packet Ins to the
controller.

Rather than separately writing the data and marking the options as present,
it is better to combine the two steps to ensure that one can never be
done without the other.

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