gianfar: Fix missing sock reference when processing TX time stamps

When there is not enough headroom in the skb a private copy will be made.
However, the private copy had no reference to the socket and consequently
no time stamp could be queued on the socket error queue during the
skb_tstamp_tx function. This patch fixes this issue by also stealing the
sock reference from the original skb after making the private copy.

Signed-off-by: Manfred Rudigier <manfred.rudigier@omicron.at>
Signed-off-by: David S. Miller <davem@davemloft.net>