extensions: libip6t_SNAT/DNAT: add square bracket in xlat output when port is specified

extensions: libip6t_SNAT/DNAT: add square bracket in xlat output when port is specified

It is better to add square brackets to ip6 address in nft translation
output when the port is specified. This is keep consistent with the
nft syntax.

Before this patch:
  # ip6tables-translate -t nat -A OUTPUT -p tcp -j DNAT --to-destination \
  [123::4]:1
  nft add rule ip6 nat OUTPUT meta l4proto tcp counter dnat to 123::4 :1
  # ip6tables-translate -t nat -A POSTROUTING -p tcp -j SNAT --to-source \
  [123::4-123::8]:1
  nft add rule ip6 nat POSTROUTING meta l4proto tcp counter snat to 123::4-123::8 :1

Apply this patch:
  # ip6tables-translate -t nat -A OUTPUT -p tcp -j DNAT --to-destination \
  [123::4]:1
  nft add rule ip6 nat OUTPUT meta l4proto tcp counter dnat to [123::4]:1
  # ip6tables-translate -t nat -A POSTROUTING -p tcp -j SNAT --to-source \
  [123::4-123::8]:1
  nft add rule ip6 nat POSTROUTING meta l4proto tcp counter snat to [123::4]-[123::8]:1

Signed-off-by: Liping Zhang <liping.zhang@spreadtrum.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>