ah: reload pointers to skb data after calling skb_cow_data()

skb_cow_data() may allocate a new data buffer, so pointers on
skb should be set after this function.

Bug was introduced by commit dff3bb06 ("ah4: convert to ahash")
and 8631e9bd ("ah6: convert to ahash").

Signed-off-by: Wang Xuefu <xuefu.wang@6wind.com>
Acked-by: Krzysztof Witek <krzysztof.witek@6wind.com>
Signed-off-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Signed-off-by: David S. Miller <davem@davemloft.net>