meta: fix a format error display when we set priority to root or none

meta: fix a format error display when we set priority to root or none

Also delete the redundant '\n'.
This fixes:

  # nft add rule filter test meta priority set root
  # nft list chain filter test
  table ip filter {
      chain test {
          meta priority set root
  none
  ffff:ffff
      }
  }

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