datapath: Backport: skbuff: Fix skb checksum partial check.

datapath: Backport: skbuff: Fix skb checksum partial check.

This bug fix is not required for OVS use cases. But is it
nice to keep function consistent with upstream implementation.

Upstream commit:

    Earlier patch 6ae459bda tried to detect void ckecksum partial
    skb by comparing pull length to checksum offset. But it does
    not work for all cases since checksum-offset depends on
    updates to skb->data.

    Following patch fixes it by validating checksum start offset
    after skb-data pointer is updated. Negative value of checksum
    offset start means there is no need to checksum.

    Fixes: 6ae459bda ("skbuff: Fix skb checksum flag on skb pull")
    Reported-by: Andrew Vagin <avagin@odin.com>
    Signed-off-by: Pravin B Shelar <pshelar@nicira.com>
    Signed-off-by: David S. Miller <davem@davemloft.net>
Upstream: 31b33dfb0a1 ("skbuff: Fix skb checksum partial check");
Signed-off-by: Pravin B Shelar <pshelar@nicira.com>
Acked-by: Jesse Gross <jesse@kernel.org>