bpftool: Fix printing incorrect pointer in btf_dump_ptr

For plain text output, it incorrectly prints the pointer value
"void *data".  The "void *data" is actually pointing to memory that
contains a bpf-map's value.  The intention is to print the content of
the bpf-map's value instead of printing the pointer pointing to the
bpf-map's value.

In this case, a member of the bpf-map's value is a pointer type.
Thus, it should print the "*(void **)data".

Fixes: 22c349e ("tools: bpftool: fix format strings and arguments for jsonw_printf()")
Signed-off-by: Martin KaFai Lau <kafai@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Reviewed-by: Quentin Monnet <quentin.monnet@netronome.com>
Link: https://lore.kernel.org/bpf/20200110231644.3484151-1-kafai@fb.com

tools/bpf/bpftool/btf_dumper.c

@@ -26,7 +26,7 @@ static void btf_dumper_ptr(const void *data, json_writer_t *jw,
 			   bool is_plain_text)
 {
 	if (is_plain_text)
-		jsonw_printf(jw, "%p", data);
+		jsonw_printf(jw, "%p", *(void **)data);
 	else
 		jsonw_printf(jw, "%lu", *(unsigned long *)data);
 }