netfilter: nft_payload: sanitize offset and length before calling skb_checksum()

In the Linux kernel, the following vulnerability has been resolved: netfilter: nft_payload: sanitize offset and length before calling skb_checksum() If access to offset + length is larger than the skbuff length, then skb_checksum() triggers BUG_ON(). skb_checksum() internally subtracts the length parameter while iterating over skbuff, BUG_ON(len) at the end of it checks that the expected length to be included in the checksum calculation is fully consumed.