bpf: Add bpf_icmp_send kfunc

This is needed in the context of Tetragon to provide improved feedback
(in contrast to just dropping packets) to east-west traffic when blocked
by policies using cgroup_skb programs.

This reuses concepts from netfilter reject target codepath with the
differences that:

* Packets are cloned since the BPF user can still let the packet pass
  (SK_PASS from the cgroup_skb progs for example) and the current skb
  need to stay untouched (cgroup_skb hooks only allow read-only skb
  payload).
* We protect against recursion since the kfunc, by generating an ICMP
  error message, could retrigger the BPF prog that invoked it.

Only ICMP_DEST_UNREACH and ICMPV6_DEST_UNREACH are currently supported.
The interface accepts a type parameter to facilitate future extension to
other ICMP control message types.

For normal cgroup_skb paths, the skb dst route should already be set.
However, bpf_prog_test_run_skb can create synthetic IPv4/IPv6 skbs
without an attached route. In that case, icmp_send returns early, and
the kfunc would otherwise report success despite no ICMP reply being
sent. This check also reject metadata dsts, which are not valid struct
rtable instances. While IPv6 would stricly require only rejecting
metadata dsts, same check is applied for API consistency.

Signed-off-by: Mahe Tardy <mahe.tardy@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Reviewed-by: Emil Tsalapatis <emil@etsalapatis.com>
Reviewed-by: Jordan Rife <jordan@jrife.io>
Acked-by: Stanislav Fomichev <sdf@fomichev.me>
Link: https://lore.kernel.org/bpf/20260709144900.245904-2-mahe.tardy@gmail.com

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