netfilter: payload_len is be16, add size of struct rather than size of pointer

payload_len is a be16 value, not cpu_endian, also the size of a ponter
to a struct ipv6hdr was being added, not the size of the struct itself.

Signed-off-by: Harvey Harrison <harvey.harrison@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>