io_uring_prep_cmd_sock: cast pointer to untptr_t first

For 32-bit archs, cast it to an uintptr_t first to avoid warnings.

Fixes: 3797db16928a ("liburing.h: Populate SQE for {s,g} etsockopt")
Signed-off-by: Jens Axboe <axboe@kernel.dk>