netfilter: xt_HL: add pr_fmt and checkentry validation

netfilter: xt_HL: add pr_fmt and checkentry validation

Add pr_fmt to prefix log messages with the module name for
easier debugging in dmesg.

Add checkentry functions for IPv4 (ttl_mt_check) and IPv6
(hl_mt6_check) to validate the match mode at rule registration
time, rejecting invalid modes with -EINVAL.

The evaluation function returns false in case the mode is
unknown, so this is a cleanup, not a bug fix.

Signed-off-by: Marino Dzalto <marino.dzalto@gmail.com>
Signed-off-by: Florian Westphal <fw@strlen.de>