macsec: enable GRO and RPS on macsec devices

Use gro_gells to trigger GRO and allow RPS on macsec traffic
after decryption.
Also, be sure to avoid clearing software offload features in
macsec_fix_features().
Overall this increase TCP tput by 30% on recent h/w.

Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Acked-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
Signed-off-by: David S. Miller <davem@davemloft.net>