gro: Change all receive functions to return GRO result codes

This will allow drivers to adjust their receive path dynamically
based on whether GRO is being applied successfully.

Currently all in-tree callers ignore the return values of these
functions and do not need to be changed.

Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>
Acked-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>