bpf: Remove unused field initialization in bpf's ctl_table

Maxlen is used by standard proc_handlers such as proc_dointvec(), but in this
case we have our own proc_handler via bpf_stats_handler(). Therefore, remove
the initialization.

Signed-off-by: Ricardo Ribalda <ribalda@chromium.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Stanislav Fomichev <sdf@google.com>
Link: https://lore.kernel.org/bpf/20221221-bpf-syscall-v1-0-9550f5f2c3fc@chromium.org