icmp: move icmp_global.credit and icmp_global.stamp to per netns storage

Host wide ICMP ratelimiter should be per netns, to provide better isolation.

Following patch in this series makes the sysctl per netns.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Link: https://patch.msgid.link/20240829144641.3880376-3-edumazet@google.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>