bpf: Fix seq_show for BPF_MAP_TYPE_STRUCT_OPS

Instead of using bpf_struct_ops_map_lookup_elem() which is
not implemented, bpf_struct_ops_map_seq_show_elem() should
also use bpf_struct_ops_map_sys_lookup_elem() which does
an inplace update to the value.  The change allocates
a value to pass to bpf_struct_ops_map_sys_lookup_elem().

[root@arch-fb-vm1 bpf]# cat /sys/fs/bpf/dctcp
{{{1}},BPF_STRUCT_OPS_STATE_INUSE,{{00000000df93eebc,00000000df93eebc},0,2, ...

Fixes: 85d33df3 ("bpf: Introduce BPF_MAP_TYPE_STRUCT_OPS")
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Martin KaFai Lau <kafai@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20200114072647.3188298-1-kafai@fb.com

kernel/bpf/bpf_struct_ops.c

@@ -496,14 +496,20 @@ static void bpf_struct_ops_map_seq_show_elem(struct bpf_map *map, void *key,
 					     struct seq_file *m)
 {
 	void *value;
+	int err;
 
-	value = bpf_struct_ops_map_lookup_elem(map, key);
+	value = kmalloc(map->value_size, GFP_USER | __GFP_NOWARN);
 	if (!value)
 		return;
 
+	err = bpf_struct_ops_map_sys_lookup_elem(map, key, value);
+	if (!err) {
 		btf_type_seq_show(btf_vmlinux, map->btf_vmlinux_value_type_id,
 				  value, m);
 		seq_puts(m, "\n");
+	}
+
+	kfree(value);
 }
 
 static void bpf_struct_ops_map_free(struct bpf_map *map)