add synflood protection

add synflood protection

Currently, a virtio-net + vhost-net can handle between 200-300 kpps for each vm (with 1core/queue=1).
That mean than a vm can easily overloaded with a simple synflood (hping3 --flood -p 80 -S targetip).
Also the conntrack of the host can be saturated easily.

This patch introduce a new option, enable rate limiting of syn/s by src ip (protection_synflood:1).

rate limit can be set with : protection_synflood_rate  (default 200 syn/s)
with an extra burst: protection_synflood_rate (default 1000).

It's also possible to reduce conntrack syn timeout: nf_conntrack_tcp_timeout_syn_recv (default 60).

with default values, a src ip can take around (60 * 200 = 12000 conntrack entries).

The iptables rules are done in raw table, before reaching the conntrack.

This protection works fine for non-spoofed src ip.
For spoofed src ip, the only way could be to implement SYNPROXY,
but this only works for routed/nat setup. (The host need to be able to reply
with the src ip the vm)

Some good information about synflood protections
https://2014.rmll.info/slides/356/day_1-1400-Jesper_Brouer-DDoS_protection_using_Netfilter_iptables.pdf

Signed-off-by: Wolfgang Bumiller <w.bumiller@proxmox.com>