clean up kernel_{read,write} & friends v2

[Christoph Hellwig]

ping?

On Wed, May 13, 2020 at 08:56:42AM +0200, Christoph Hellwig wrote:
> Hi Al,
> 
> this series fixes a few issues and cleans up the helpers that read from
> or write to kernel space buffers, and ensures that we don't change the
> address limit if we are using the ->read_iter and ->write_iter methods
> that don't need the changed address limit.
> 
> Changes since v1:
>  - __kernel_write must not take sb_writers
>  - unexported __kernel_write
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