[PATCH 01/10] fs: don't allow kernel reads and writes without iter ops

['Christoph Hellwig']

On Thu, Aug 27, 2020 at 03:58:02PM +0000, David Laight wrote:
> Is there a real justification for that?
> For system calls supplying both methods makes sense to avoid
> the extra code paths for a simple read/write.

Al asked for it as two of our four in-tree instances do have weird
semantics, and we can't change that any more.  And the other two
don't make sense to be used with kernel_read and kernel_write (
(/dev/null and /dev/zero).