net: optimize ICMP-reply code path

This patchset is optimizing the ICMP-reply code path, for ICMP packets that gets rate limited. A remote party can easily trigger this code path by sending packets to port number with no listening service.

Generally the patchset moves the sysctl_icmp_msgs_per_sec ratelimit checking to earlier in the code path and removes an allocation.

Use-case: The specific case I experienced this being a bottleneck is, sending UDP packets to a port with no listener, which obviously result in kernel replying with ICMP Destination Unreachable (type:3), Port Unreachable (code:3), which cause the bottleneck.

After Eric and Paolo optimized the UDP socket code, the kernels PPS
processing capabilities is lower for no-listen ports, than normal UDP sockets. This is bad for capacity planning when restarting a service.

UDP no-listen benchmark 8xCPUs using pktgen_sample04_many_flows.sh:
Baseline: 6.6 Mpps
Patch: 14.7 Mpps
Driver mlx5 at 50Gbit/s.

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Jesper Dangaard Brouer (3):
Revert "icmp: avoid allocating large struct on stack"
net: reduce cycles spend on ICMP replies that gets rate limited
net: for rate-limited ICMP replies save one atomic operation

 net/ipv4/icmp.c |  125 +++++++++++++++++++++++++++++++++----------------------
 net/ipv6/icmp.c |   68 +++++++++++++++++++++---------
 2 files changed, 123 insertions(+), 70 deletions(-)