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 <[email protected]>
Signed-off-by: Daniel Borkmann <[email protected]>
Tested-by: Eduard Zingerman <[email protected]>
Acked-by: Eduard Zingerman <[email protected]>
Acked-by: Quentin Monnet <[email protected]>
Link: https://lore.kernel.org/bpf/[email protected]

0 files changed, 0 insertions, 0 deletions