vringh: replace kmap_atomic() with kmap_local_page()

vringh: replace kmap_atomic() with kmap_local_page()

kmap_atomic() is deprecated in favor of kmap_local_page() since commit
f3ba3c710ac5 ("mm/highmem: Provide kmap_local*").

With kmap_local_page() the mappings are per thread, CPU local, can take
page-faults, and can be called from any context (including interrupts).
Furthermore, the tasks can be preempted and, when they are scheduled to
run again, the kernel virtual addresses are restored and still valid.

kmap_atomic() is implemented like a kmap_local_page() which also disables
page-faults and preemption (the latter only for !PREEMPT_RT kernels,
otherwise it only disables migration).

The code within the mappings/un-mappings in getu16_iotlb() and
putu16_iotlb() don't depend on the above-mentioned side effects of
kmap_atomic(), so that mere replacements of the old API with the new one
is all that is required (i.e., there is no need to explicitly add calls
to pagefault_disable() and/or preempt_disable()).

This commit reuses a "boiler plate" commit message from Fabio, who has
already did this change in several places.

Cc: "Fabio M. De Francesco" <fmdefrancesco@gmail.com>
Reviewed-by: Fabio M. De Francesco <fmdefrancesco@gmail.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
Message-Id: <20230404131326.44403-4-sgarzare@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>