gro: Move common completion code into helpers

Currently VLAN still has a bit of common code handling the aftermath
of GRO that's shared with the common path.  This patch moves them
into shared helpers to reduce code duplication.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>