ipv4: icmp: icmpv4_xrlim_allow() optimization if net.ipv4.icmp_ratelimit is zero

If net.ipv4.icmp_ratelimit is zero, we do not have to call
inet_getpeer_v4() and inet_peer_xrlim_allow().

Both can be very expensive under DDOS.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Kuniyuki Iwashima <kuniyu@google.com>
Link: https://patch.msgid.link/20260216142832.3834174-5-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>