[NETFILTER] NAT sequence adjustment: Save eight bytes per conntrack

This patch reduces the size of 'struct ip_conntrack' on systems with NAT
by eight bytes.  The sequence number delta values can be int16_t, since
we only support one sequence number modification per window anyway, and
one such modification is not going to exceed 32kB ;)

Signed-off-by: Harald Welte <laforge@netfilter.org>
Signed-off-by: David S. Miller <davem@davemloft.net>