net: stmmac: free an skb first when there are no longer any descriptors using it

When having the skb pointer in the first descriptor, stmmac_tx_clean
can get called at a moment where the IP has only cleared the own bit
of the first descriptor, thus freeing the skb, even though there can
be several descriptors whose buffers point into the same skb.

By simply moving the skb pointer from the first descriptor to the last
descriptor, a skb will get freed only when the IP has cleared the
own bit of all the descriptors that are using that skb.

Signed-off-by: Niklas Cassel <niklas.cassel@axis.com>
Signed-off-by: David S. Miller <davem@davemloft.net>