selftests/bpf: fix constness of source arg for bpf helpers

Fix signature of bpf_probe_read and bpf_probe_write_user to mark source
pointer as const. This causes warnings during compilation for
applications relying on those helpers.

Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Acked-by: Martin KaFai Lau <kafai@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>