netfilter: nft_log: restrict the log prefix length to 127

First, log prefix will be truncated to NF_LOG_PREFIXLEN-1, i.e. 127,
at nf_log_packet(), so the extra part is useless.

Second, after adding a log rule with a very very long prefix, we will
fail to dump the nft rules after this _special_ one, but acctually,
they do exist. For example:
  # name_65000=$(printf "%0.sQ" {1..65000})
  # nft add rule filter output log prefix "$name_65000"
  # nft add rule filter output counter
  # nft add rule filter output counter
  # nft list chain filter output
  table ip filter {
      chain output {
          type filter hook output priority 0; policy accept;
      }
  }

So now, restrict the log prefix length to NF_LOG_PREFIXLEN-1.

Fixes: 96518518 ("netfilter: add nftables")
Signed-off-by: Liping Zhang <zlpnobody@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>