gianfar: fix potential sk_wmem_alloc imbalance

commit db83d136 (gianfar: Fix missing sock reference when
processing TX time stamps) added a potential sk_wmem_alloc imbalance

If the new skb has a different truesize than old one, we can get a
negative sk_wmem_alloc once new skb is orphaned at TX completion.

Now we no longer early orphan skbs in dev_hard_start_xmit(), this
probably can lead to fatal bugs.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Tested-by: Paul Gortmaker <paul.gortmaker@windriver.com>
Cc: Manfred Rudigier <manfred.rudigier@omicron.at>
Cc: Claudiu Manoil <claudiu.manoil@freescale.com>
Cc: Jiajun Wu <b06378@freescale.com>
Cc: Andy Fleming <afleming@freescale.com>
Signed-off-by: David S. Miller <davem@davemloft.net>