For SNAT, don't store the pre-fragment L2 header before actions are applied.

For SNAT, don't store the pre-fragment L2 header before actions are applied.

The IP fragment code doesn't always write the L2 header when generating
new fragments.  This problem was fixed in an earlier commit.
Unfortunately, we stored the pre-fragment L2 header when the packet
first arrived--before other packet modifications were applied.  This
meant that the results of any OpenFlow L2 modification actions were lost.
This patch pushes the storage of the L2 header until right before the
packet is transmitted (and possibly refragmented).

Thanks to Dan for catching this behavior.