skbuff: Copy csum instead of csum_start/csum_offset

Hi:

skbuff: Copy csum instead of csum_start/csum_offset

It's easier to copy the u32 csum instead of its two u16
constituents.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

Cheers,
Signed-off-by: David S. Miller <davem@davemloft.net>