net: lan78xx: Fix race in tx skb size calculation (#2614)

The skb size calculation in lan78xx_tx_bh is in race with the start_xmit,
which could lead to kernel oopses. So protect the whole skb walk with
a spin lock. As a benefit we can unlink the skb directly.

See: https://github.com/raspberrypi/linux/issues/2608



Signed-off-by: Floris Bos <bos@je-eigen-domein.nl>
Signed-off-by: Stefan Wahren <stefan.wahren@i2se.com>
Fixes: 55d7de9d ("Microchip's LAN7800 family USB 2/3 to 10/100/1000 Ethernet")