netfilter: nf_conntrack_ipv6: fix nf_log_packet message in icmpv6 conntrack

This patch fixes a trivial typo that was adding a new line at end of
the nf_log_packet() prefix. It also make the logging conditionnal by
adding a LOG_INVALID test.

Signed-off-by: Eric Leblond <eric@inl.fr>
Signed-off-by: Patrick McHardy <kaber@trash.net>