sfc: check for zero length in EF10 RX prefix

When EF10 RXDP firmware is operating in cut-through mode, packet length
 is not known at the time the RX prefix is generated, so it is left as
 zero and RX event merging is inhibited to ensure that the length is
 available in the RX event.  However, it has been found that in certain
 circumstances the RX events for these packets still get merged,
 meaning the driver cannot read the length from the RX event, and tries
 to use the length from the prefix.
The resulting zero-length SKBs cause crashes in GRO since commit
 1d11fa69 ("net-gro: remove GRO_DROP"), so add a check to the driver
 to detect these zero-length RX events and discard the packet.

Signed-off-by: Edward Cree <ecree.xilinx@gmail.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>