tools/bpftool: show btf_id in map listing

Let's print btf id of map similar to the way we are printing it
for programs.

Sample output:
user@test# bpftool map -f
61: lpm_trie flags 0x1
key 20B value 8B max_entries 1 memlock 4096B
133: array name test_btf_id flags 0x0
key 4B value 4B max_entries 4 memlock 4096B
pinned /sys/fs/bpf/test100
btf_id 174
170: array name test_btf_id flags 0x0
key 4B value 4B max_entries 4 memlock 4096B
btf_id 240

Signed-off-by: Prashant Bhole <[e-mail]>
Reviewed-by: Quentin Monnet <[e-mail]>
Reviewed-by: Jakub Kicinski <[e-mail]>
Acked-by: Song Liu <[e-mail]>
Signed-off-by: Alexei Starovoitov <[e-mail]>

tools/bpf/bpftool/map.c

···
 	}
 	close(fd);
 
+	if (info->btf_id)
+		jsonw_int_field(json_wtr, "btf_id", info->btf_id);
+
 	if (!hash_empty(map_table.table)) {
 		struct pinned_obj *obj;
 
···
 				printf("\n\tpinned %s", obj->path);
 		}
 	}
+
+	if (info->btf_id)
+		printf("\n\tbtf_id %d", info->btf_id);
 
 	printf("\n");
 	return 0;