netfilter: conntrack: do not increment two error counters at same time

The /proc interface for nf_conntrack displays the "error" counter as
"icmp_error".

It makes sense to not increment "invalid" when failing to handle an icmp
packet since those are special.

For example, its possible for conntrack to see partial and/or fragmented
packets inside icmp errors.  This should be a separate event and not get
mixed with the "invalid" counter.

Likewise, remove the "error" increment for errors from get_l4proto().
After this, the error counter will only increment for errors coming from
icmp(v6) packet handling.

Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>