s390/qeth: drop unwanted packets earlier in RX path

Packets with an unexpected HW format are currently first extracted from
the RX buffer, passed upwards to the layer-specific driver and only then
finally dropped.

Enhance the RX path so that we can drop such packets before even
allocating an skb. For this, add some additional logic so that when a
packet is meant to be dropped, we can still walk along the packet's data
chunks in the RX buffer. This allows us to extract the following
packet(s) from the buffer.

Signed-off-by: Julian Wiedmann <jwi@linux.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>