bpf: return EOPNOTSUPP when map lookup isn't supported

Return ERR_PTR(-EOPNOTSUPP) from map_lookup_elem() methods of below
map types:
- BPF_MAP_TYPE_PROG_ARRAY
- BPF_MAP_TYPE_STACK_TRACE
- BPF_MAP_TYPE_XSKMAP
- BPF_MAP_TYPE_SOCKMAP/BPF_MAP_TYPE_SOCKHASH

Signed-off-by: Prashant Bhole <bhole_prashant_q7@lab.ntt.co.jp>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Song Liu <songliubraving@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

kernel/bpf/arraymap.c

@@ -449,7 +449,7 @@ static void fd_array_map_free(struct bpf_map *map)
 
 static void *fd_array_map_lookup_elem(struct bpf_map *map, void *key)
 {
-	return NULL;
+	return ERR_PTR(-EOPNOTSUPP);
 }
 
 /* only called from syscall */

kernel/bpf/sockmap.c

@@ -2096,7 +2096,7 @@ int sockmap_get_from_fd(const union bpf_attr *attr, int type,
 
 static void *sock_map_lookup(struct bpf_map *map, void *key)
 {
-	return NULL;
+	return ERR_PTR(-EOPNOTSUPP);
 }
 
 static int sock_map_update_elem(struct bpf_map *map,

kernel/bpf/stackmap.c

@@ -505,7 +505,7 @@ const struct bpf_func_proto bpf_get_stack_proto = {
 /* Called from eBPF program */
 static void *stack_map_lookup_elem(struct bpf_map *map, void *key)
 {
-	return NULL;
+	return ERR_PTR(-EOPNOTSUPP);
 }
 
 /* Called from syscall */

kernel/bpf/xskmap.c

@@ -154,7 +154,7 @@ void __xsk_map_flush(struct bpf_map *map)
 
 static void *xsk_map_lookup_elem(struct bpf_map *map, void *key)
 {
-	return NULL;
+	return ERR_PTR(-EOPNOTSUPP);
 }
 
 static int xsk_map_update_elem(struct bpf_map *map, void *key, void *value,