[NETFILTER]: Use HOPLIMIT metric as TTL of TCP reset sent by REJECT

HOPLIMIT metric is appropriate to TCP reset sent by REJECT target
than hard-coded max TTL. Thanks to David S. Miller for hint.

Signed-off-by: Yasuyuki Kozakai <yasuyuki.kozakai@toshiba.co.jp>
Signed-off-by: David S. Miller <davem@davemloft.net>