ip, link: Add support for netkit

ip, link: Add support for netkit

Add base support for creating/dumping netkit devices.

Minimal example usage:

  # ip link add type netkit
  # ip -d a
  [...]
  7: nk0@nk1: <BROADCAST,MULTICAST,NOARP,M-DOWN> mtu 1500 qdisc noop state DOWN group default qlen 1000
    link/ether 00:00:00:00:00:00 brd ff:ff:ff:ff:ff:ff promiscuity 0 allmulti 0 minmtu 68 maxmtu 65535
    netkit mode l3 type peer policy forward numtxqueues 1 numrxqueues 1 [...]
  8: nk1@nk0: <BROADCAST,MULTICAST,NOARP,M-DOWN> mtu 1500 qdisc noop state DOWN group default qlen 1000
    link/ether 00:00:00:00:00:00 brd ff:ff:ff:ff:ff:ff promiscuity 0 allmulti 0 minmtu 68 maxmtu 65535
    netkit mode l3 type primary policy forward numtxqueues 1 numrxqueues 1 [...]

Example usage with netns (for BPF examples, see BPF selftests linked below):

  # ip netns add blue
  # ip link add nk0 type netkit peer nk1 netns blue
  # ip link set up nk0
  # ip addr add 10.0.0.1/24 dev nk0
  # ip -n blue link set up nk1
  # ip -n blue addr add 10.0.0.2/24 dev nk1
  # ping -c1 10.0.0.2
  PING 10.0.0.2 (10.0.0.2) 56(84) bytes of data.
  64 bytes from 10.0.0.2: icmp_seq=1 ttl=64 time=0.021 ms

Example usage with L2 mode and peer blackholing when no BPF is attached:

  # ip link add foo type netkit mode l2 forward peer blackhole bar
  # ip -d a
  [...]
  13: bar@foo: <BROADCAST,MULTICAST,M-DOWN> mtu 1500 qdisc noop state DOWN group default qlen 1000
     link/ether 5e:5b:81:17:02:27 brd ff:ff:ff:ff:ff:ff promiscuity 0 allmulti 0 minmtu 68 maxmtu 65535
     netkit mode l2 type peer policy blackhole numtxqueues 1 numrxqueues 1 [...]
  14: foo@bar: <BROADCAST,MULTICAST,M-DOWN> mtu 1500 qdisc noop state DOWN group default qlen 1000
     link/ether de:01:a5:88:9e:99 brd ff:ff:ff:ff:ff:ff promiscuity 0 allmulti 0 minmtu 68 maxmtu 65535
     netkit mode l2 type primary policy forward numtxqueues 1 numrxqueues 1 [...]

Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Reviewed-by: Nikolay Aleksandrov <razor@blackwall.org>
Link: https://git.kernel.org/torvalds/c/35dfaad7188c
Link: https://git.kernel.org/torvalds/c/05c31b4ab205
Link: https://git.kernel.org/torvalds/c/ace15f91e569
Signed-off-by: David Ahern <dsahern@kernel.org>