net: stmmac: fix memory corruption with large MTUs

net: stmmac: fix memory corruption with large MTUs

When using 16K DMA buffers and ring mode, the DES3 refill is not working
correctly as the function is using a bogus pointer for checking the
private data. As a result stale pointers will remain in the RX descriptor
ring, so DMA will now likely overwrite/corrupt some already freed memory.

As simple reproducer, just receive some UDP traffic:

	# ifconfig eth0 down; ifconfig eth0 mtu 9000; ifconfig eth0 up
	# iperf3 -c 192.168.253.40 -u -b 0 -R

If you didn't crash by now check the RX descriptors to find non-contiguous
RX buffers:

	cat /sys/kernel/debug/stmmaceth/eth0/descriptors_status
	[...]
	1 [0x2be5020]: 0xa3220321 0x9ffc1ffc 0x72d70082 0x130e207e
					     ^^^^^^^^^^^^^^^^^^^^^
	2 [0x2be5040]: 0xa3220321 0x9ffc1ffc 0x72998082 0x1311a07e
					     ^^^^^^^^^^^^^^^^^^^^^

A simple ping test will now report bad data:

	# ping -s 8200 192.168.253.40
	PING 192.168.253.40 (192.168.253.40) 8200(8228) bytes of data.
	8208 bytes from 192.168.253.40: icmp_seq=1 ttl=64 time=1.00 ms
	wrong data byte #8144 should be 0xd0 but was 0x88

Fix the wrong pointer. Also we must refill DES3 only if the DMA buffer
size is 16K.

Fixes: 54139cf3bb33 ("net: stmmac: adding multiple buffers for rx")
Signed-off-by: Aaro Koskinen <aaro.koskinen@nokia.com>
Acked-by: Jose Abreu <joabreu@synopsys.com>
Signed-off-by: David S. Miller <davem@davemloft.net>