extensions: libxt_NFLOG: don't truncate log prefix on print/save

extensions: libxt_NFLOG: don't truncate log prefix on print/save

When parsing the rule, use a struct with a layout compatible to that of
struct xt_nflog_info, but with a buffer large enough to contain the
whole 128-character nft prefix.

We always send the nflog-group to the kernel since, for nft, log and
nflog targets are handled by the same kernel module, and are
distinguished by whether they define an nflog-group. Therefore, we must
send the group even if it is zero, or the kernel will configure the
target as a log, not an nflog.

Changes to nft_is_expr_compatible were made since only targets which
have an `nflog-group` are compatible. Since nflog targets are
distinguished by having an nflog-group, we ignore targets without one.

We also set the copy-len flag if the snap-len is set since without this,
iptables will mistake `nflog-size` for `nflog-range`.

Signed-off-by: Kyle Bowman <kbowman@cloudflare.com>
Signed-off-by: Alex Forster <aforster@cloudflare.com>
Signed-off-by: Jeremy Sowden <jeremy@azazel.net>
Signed-off-by: Florian Westphal <fw@strlen.de>