[NETFILTER]: Fix check whether dst_entry needs to be released after NAT

After DNAT the original dst_entry needs to be released if present
so the packet doesn't skip input routing with its new address. The
current check for DNAT in ip_nat_in is reversed and checks for SNAT.

Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>