meta: fix endianness in priority

meta: fix endianness in priority

For example, after we add rule to set priority 1:2, it will be displayed in network
byte order as 0200:0100, this is wrong:

  # nft add rule filter test meta priority set 1:2
  # nft list chain filter test
  table ip filter {
      chain test {
          meta priority set 0200:0100
      }
  }

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