[NETFILTER]: nf_{conntrack,nat}_icmp: constify and annotate

Constify a few data tables use const qualifiers on variables where
possible in the nf_conntrack_icmp* sources.

Signed-off-by: Jan Engelhardt <jengelh@computergmbh.de>
Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>