net: gro: Fix a 'directive in macro's argument list' sparse warning

Following the cited commit, sparse started complaining about:
../include/net/gro.h:58:1: warning: directive in macro's argument list
../include/net/gro.h:59:1: warning: directive in macro's argument list

Fix that by moving the defines out of the struct_group() macro.

Fixes: de5a1f3c ("net: gro: minor optimization for dev_gro_receive()")
Reviewed-by: Maxim Mikityanskiy <maximmi@nvidia.com>
Signed-off-by: Gal Pressman <gal@nvidia.com>
Acked-by: Alexander Lobakin <alexandr.lobakin@intel.com>
Acked-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>