bpf: create samples/bpf/tcp_bpf.readme

Readme file explaining how to create a cgroupv2 and attach one
of the tcp_*_kern.o socket_ops BPF program.
Signed-off-by: Lawrence Brakmo <brakmo@fb.com>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Acked_by: Alexei Starovoitov <ast@fb.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

samples/bpf/tcp_bbf.readme

+This file describes how to run the tcp_*_kern.o tcp_bpf (or socket_ops)
+programs. These programs attach to a cgroupv2. The following commands create
+a cgroupv2 and attach a bash shell to the group.
+
+  mkdir -p /tmp/cgroupv2
+  mount -t cgroup2 none /tmp/cgroupv2
+  mkdir -p /tmp/cgroupv2/foo
+  bash
+  echo $$ >> /tmp/cgroupv2/foo/cgroup.procs
+
+Anything that runs under this shell belongs to the foo cgroupv2 To load
+(attach) one of the tcp_*_kern.o programs:
+
+  ./load_sock_ops -l /tmp/cgroupv2/foo tcp_basertt_kern.o
+
+If the "-l" flag is used, the load_sock_ops program will continue to run
+printing the BPF log buffer. The tcp_*_kern.o programs use special print
+functions to print logging information (if enabled by the ifdef).
+
+If using netperf/netserver to create traffic, you need to run them under the
+cgroupv2 to which the BPF programs are attached (i.e. under bash shell
+attached to the cgroupv2).
+
+To remove (unattach) a socket_ops BPF program from a cgroupv2:
+
+  ./load_sock_ops -r /tmp/cgroupv2/foo