bpf: cpumap: switch to GRO from netif_receive_skb_list()

cpumap has its own BH context based on kthread. It has a sane batch
size of 8 frames per one cycle.
GRO can be used here on its own. Adjust cpumap calls to the upper stack
to use GRO API instead of netif_receive_skb_list() which processes skbs
by batches, but doesn't involve GRO layer at all.
In plenty of tests, GRO performs better than listed receiving even
given that it has to calculate full frame checksums on the CPU.
As GRO passes the skbs to the upper stack in the batches of
@gro_normal_batch, i.e. 8 by default, and skb->dev points to the
device where the frame comes from, it is enough to disable GRO
netdev feature on it to completely restore the original behaviour:
untouched frames will be being bulked and passed to the upper stack
by 8, as it was with netif_receive_skb_list().

Tested-by: Daniel Xu <dxu@dxuuu.xyz>
Reviewed-by: Toke Høiland-Jørgensen <toke@redhat.com>
Signed-off-by: Alexander Lobakin <aleksander.lobakin@intel.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>