sfc: Set ip_summed correctly for page buffers passed to GRO

Page buffers containing packets with an incorrect checksum or using a
protocol not handled by hardware checksum offload were previously not
passed to LRO.  The conversion to GRO changed this, but did not set
the ip_summed value accordingly.

Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>