let import_iovec deal with compat_iovecs as well

[David Laight]

From: Christoph Hellwig
> Sent: 18 September 2020 13:45
> 
> this series changes import_iovec to transparently deal with comat iovec
> structures, and then cleanups up a lot of code dupliation.  But to get
> there it first has to fix the pre-existing bug that io_uring compat
> contexts don't trigger the in_compat_syscall() check.  This has so far
> been relatively harmless as very little code callable from io_uring used
> the check, and even that code that could be called usually wasn't.

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?
	David

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