gianfar: reallocate skb when headroom is not enough for fcb

Gianfar uses a hardware header FCB for offloading.  However when used
with bridging or IP forwarding, TX skb might not have enough headroom
for the FCB.  Reallocate skb for such cases.

Signed-off-by: Li Yang <leoli@freescale.com>
Signed-off-by: David S. Miller <davem@davemloft.net>