bpf, samples: Use bpf_probe_read_user where appropriate

Use bpf_probe_read_user() helper instead of bpf_probe_read() for samples that
attach to kprobes probing on user addresses.

Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Andrii Nakryiko <andriin@fb.com>
Link: https://lore.kernel.org/bpf/5b0144b3f8e031ec5e2438bd7de8d7877e63bf2f.1572649915.git.daniel@iogearbox.net