gro: Open-code memcpy in napi_fraginfo_skb

This patch optimises napi_fraginfo_skb to only copy the bits
necessary.  We also open-code the memcpy so that the alignment
information is always available to gcc.

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