let import_iovec deal with compat_iovecs as well

['Christoph Hellwig']

On Sat, Sep 19, 2020 at 02:24:10PM +0000, David Laight wrote:
> I thought about that change while writing my import_iovec() => iovec_import()
> patch - and thought that the io_uring code would (as usual) cause grief.
> 
> Christoph - did you see those patches?

No.