cxgb3 - Unmap offload packets when they are freed

Offload packets may be DMAed long after their SGE Tx descriptors are done
so they must remain mapped until they are freed rather than until their
descriptors are freed.  Unmap such packets through an skb destructor.

Signed-off-by: Divy Le Ray <divy@chelsio.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>