icmp: prevent possible overflow in icmp_global_allow()

Following expression can overflow
if sysctl_icmp_msgs_per_sec is big enough.

sysctl_icmp_msgs_per_sec * delta / HZ;

Fixes: 4cdf507d5452 ("icmp: add a global rate limitation")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Kuniyuki Iwashima <kuniyu@google.com>
Link: https://patch.msgid.link/20260216142832.3834174-2-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>