[PATCH bpf-next v3] bpftool: Dump map id instead of value for map_of_maps types

[Xueming Feng <kuro@kuroa.me>]

When using `bpftool map dump` with map_of_maps, it is usually
more convenient to show the inner map id instead of raw value.

We are changing the plain print behavior to show inner_map_id
instead of hex value, this would help with quick look up of
inner map with `bpftool map dump id <inner_map_id>`.
To avoid disrupting scripted behavior, we will add a new
`inner_map_id` field to json output instead of replacing value.

plain print:
```
$ bpftool map dump id 138

Without Patch:
key:
fc 00 00 00 00 00 00 00  00 00 00 00 00 00 00 05
27 16 06 00
value:
8b 00 00 00
Found 1 element

With Patch:
key:
fc 00 00 00 00 00 00 00  00 00 00 00 00 00 00 05
27 16 06 00
inner_map_id:
139
Found 1 element
```

json print:
```
$ bpftool -p map dump id 567

Without Patch:
[{
        "key": ["0xc0","0x00","0x02","0x05","0x27","0x16","0x06","0x00"
        ],
        "value": ["0x38","0x02","0x00","0x00"
        ]
    }
]

With Patch:
[{
        "key": ["0xc0","0x00","0x02","0x05","0x27","0x16","0x06","0x00"
        ],
        "value": ["0x38","0x02","0x00","0x00"
        ],
        "inner_map_id": 568
    }
]
```

Signed-off-by: Xueming Feng <kuro@kuroa.me>
---

Changes in v3:
 - In plain print, use printf() directly since inner map id is always a 32bit int.
 - Remove unused print_uint() function.
 - Rename `id` to `inner_map_id` in plain print output for clearness.
 - Add a new `inner_map_id` field to json output.
 - Add example output to commit message.

Changes in v2:
 - Fix commit message grammar.
 - Change `print_uint` to only print to stdout, make `arg` const, 
   and rename `n` to `arg_size`.
 - Make `print_uint` able to take any size of argument up to `unsigned long`, 
   and print it as unsigned decimal.

Sorry for taking this long, my compiling machine is acting weird. And thanks for
the reviews and suggestions!

 tools/bpf/bpftool/map.c | 13 +++++++++++--
 1 file changed, 11 insertions(+), 2 deletions(-)

diff --git a/tools/bpf/bpftool/map.c b/tools/bpf/bpftool/map.c
index aaeb8939e137..afcd8455db74 100644
--- a/tools/bpf/bpftool/map.c
+++ b/tools/bpf/bpftool/map.c
@@ -139,6 +139,10 @@ static void print_entry_json(struct bpf_map_info *info, unsigned char *key,
 		print_hex_data_json(key, info->key_size);
 		jsonw_name(json_wtr, "value");
 		print_hex_data_json(value, info->value_size);
+		if (map_is_map_of_maps(info->type)) {
+			jsonw_uint_field(json_wtr, "inner_map_id",
+					 *(unsigned int *)value);
+		}
 		if (btf) {
 			struct btf_dumper d = {
 				.btf = btf,
@@ -259,8 +263,13 @@ static void print_entry_plain(struct bpf_map_info *info, unsigned char *key,
 		}
 
 		if (info->value_size) {
-			printf("value:%c", break_names ? '\n' : ' ');
-			fprint_hex(stdout, value, info->value_size, " ");
+			if (map_is_map_of_maps(info->type)) {
+				printf("inner_map_id:%c", break_names ? '\n' : ' ');
+				printf("%u ", *(unsigned int *)value);
+			} else {
+				printf("value:%c", break_names ? '\n' : ' ');
+				fprint_hex(stdout, value, info->value_size, " ");
+			}
 		}
 
 		printf("\n");
-- 
2.37.1 (Apple Git-137.1)