skb->len is used to calculate statistics after xmit invocation.

Under a stress load it may happen that skb will be xmited,
rx interrupt will come and skb will be freed, all before xmit function
is even returned.

Eventually, skb->len will access unallocated area.

Moving stats calculation into tx_clean routine.

Fixes: 018423e9 ("net: ethernet: aquantia: Add ring support code")
Reported-by: Christophe Vu-Brugier <cvubrugier@fastmail.fm>
Signed-off-by: Igor Russkikh <irusskikh@marvell.com>
Signed-off-by: Pavel Belous <pbelous@marvell.com>
Signed-off-by: Dmitry Bogdanov <dbogdanov@marvell.com>
Signed-off-by: David S. Miller <davem@davemloft.net>