netlink: only display wanted chain in listing

netlink: only display wanted chain in listing

When specifying a chain to list, all created chains were displayed
with a void content:

 # nft list chain filter
 table ip filter {
  	chain input {
  	}

  	chain new {
  		 counter packets 17971 bytes 2380637 accept
  		 counter packets 0 bytes 0 accept
  	}
  }

With the attached patch, only the asked chain is displayed:

  # nft list chain filter
  table ip filter {
  	chain new {
  		 counter packets 17971 bytes 2380637 accept # handle 36
  		 counter packets 0 bytes 0 accept # handle 40
  	}
  }

Signed-off-by: Eric Leblond <eric@regit.org>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>