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>