bpftool: Fix printing of pointer value

When printing a pointer value, "%p" will either print the hexadecimal
value of the pointer (e.g `0x1234`), or `(nil)` when NULL.

Both of those are invalid json "integer" values and need to be wrapped
in quotes.

Before:
```
$ sudo bpftool struct_ops dump  name ned_dummy_cca | grep next
                    "next": (nil),
$ sudo bpftool struct_ops dump  name ned_dummy_cca | \
    jq '.[1].bpf_struct_ops_tcp_congestion_ops.data.list.next'
parse error: Invalid numeric literal at line 29, column 34
```

After:
```
$ sudo ./bpftool struct_ops dump  name ned_dummy_cca | grep next
                    "next": "(nil)",
$ sudo ./bpftool struct_ops dump  name ned_dummy_cca | \
    jq '.[1].bpf_struct_ops_tcp_congestion_ops.data.list.next'
"(nil)"
```

Signed-off-by: Manu Bretelle <chantr4@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Tested-by: Eduard Zingerman <eddyz87@gmail.com>
Acked-by: Eduard Zingerman <eddyz87@gmail.com>
Acked-by: Quentin Monnet <quentin@isovalent.com>
Link: https://lore.kernel.org/bpf/20231018230133.1593152-2-chantr4@gmail.com