VFS: AF_UNIX sockets should call mknod on the top layer only

AF_UNIX sockets should call mknod on the top layer only and should not attempt
to modify the lower layer in a layered filesystem such as overlayfs.

Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>