vfio/ccw: replace copy_from_iova with vfio_dma_rw

vfio/ccw: replace copy_from_iova with vfio_dma_rw

It was suggested [1] that we replace the old copy_from_iova() routine
(which pins a page, does a memcpy, and unpins the page) with the
newer vfio_dma_rw() interface.

This has a modest improvement in the overall time spent through the
fsm_io_request() path, and simplifies some of the code to boot.

[1] https://lore.kernel.org/r/20220706170553.GK693670@nvidia.com/

Suggested-by: Jason Gunthorpe <jgg@nvidia.com>
Signed-off-by: Eric Farman <farman@linux.ibm.com>
Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Reviewed-by: Matthew Rosato <mjrosato@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>