[AF_UNIX]: Use lookup_create().

currently it opencodes it, but that's in the way of chaning the
lookup_hash interface.

I'd prefer to disallow modular af_unix over exporting lookup_create,
but I'll leave that to you.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: David S. Miller <davem@davemloft.net>