netfilter: xt_connlimit: pick right dstaddr in NAT scenario

xt_connlimit normally records the "original" tuples in a hashlist
(such as "1.2.3.4 -> 5.6.7.8"), and looks in this list for iph->daddr
when counting.

When the user however uses DNAT in PREROUTING, looking for
iph->daddr -- which is now 192.168.9.10 -- will not match. Thus in
daddr mode, we need to record the reverse direction tuple
("192.168.9.10 -> 1.2.3.4") instead. In the reverse tuple, the dst
addr is on the src side, which is convenient, as count_them still uses
&conn->tuple.src.u3.

Signed-off-by: Jan Engelhardt <jengelh@medozas.de>