io_uring: wire up allocated direct descriptors for socket

The socket support was merged in an earlier branch that didn't yet
have support for allocating direct descriptors, hence only open
and accept got support for that.

Do the one-liner to enable it now, so we have consistent support for
any request that can instantiate a file/direct descriptor.

Reviewed-by: Hao Xu <howeyxu@tencent.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>