ipv6: Set default hoplimit as zero.

Commit a02e4b7dae4551(Demark default hoplimit as zero) only changes the
hoplimit checking condition and default value in ip6_dst_hoplimit, not
zeros all hoplimit default value.

Keep the zeroing ip6_template_metrics[RTAX_HOPLIMIT - 1] to force it as
const, cause as a37e6e344910(net: force dst_default_metrics to const
section)

Signed-off-by: Li RongQing <roy.qing.li@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>