net: Handle CHECKSUM_COMPLETE more adequately in pskb_trim_rcsum().

Currently pskb_trim_rcsum() just balks on CHECKSUM_COMPLETE packets
and remarks them as CHECKSUM_NONE, forcing a software checksum
validation later.

We have all of the mechanics available to fixup the skb->csum value,
even for complicated fragmented packets, via the helpers
skb_checksum() and csum_sub().

So just use them.

Based upon a suggestion by Herbert Xu.

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