add a bvec_virt helper

[Christoph Hellwig]

ping.

On Wed, Aug 04, 2021 at 11:56:19AM +0200, Christoph Hellwig wrote:
> Hi Jens,
> 
> this series adds a bvec_virt helper to return the virtual address of the
> data in bvec to replace the open coded calculation, and as a reminder
> that generall bio/bvec data can be in high memory unless it is caller
> controller or in an architecture specific driver where highmem is
> impossible.
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