ceph: strengthen rsize/wsize/readdir_max_bytes validation

The check (intval < PAGE_SIZE) will involve type cast, so even when
specifying negative value to rsize/wsize/readdir_max_bytes, it will
pass the validation check successfully.

Signed-off-by: Chengguang Xu <cgxu519@gmx.com>
Reviewed-by: Ilya Dryomov <idryomov@gmail.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>