bpf: Do not allow map_freeze in struct_ops map

struct_ops map cannot support map_freeze.  Otherwise, a struct_ops
cannot be unregistered from the subsystem.

Fixes: 85d33df3 ("bpf: Introduce BPF_MAP_TYPE_STRUCT_OPS")
Signed-off-by: Martin KaFai Lau <kafai@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20200305013454.535397-1-kafai@fb.com

kernel/bpf/syscall.c

@@ -1510,6 +1510,11 @@ static int map_freeze(const union bpf_attr *attr)
 	if (IS_ERR(map))
 		return PTR_ERR(map);
 
+	if (map->map_type == BPF_MAP_TYPE_STRUCT_OPS) {
+		fdput(f);
+		return -ENOTSUPP;
+	}
+
 	mutex_lock(&map->freeze_mutex);
 
 	if (map->writecnt) {