swiotlb-xen: ensure to issue well-formed XENMEM_exchange requests

While the hypervisor hasn't been enforcing this, we would still better
avoid issuing requests with GFNs not aligned to the requested order.
Instead of altering the value also in the call to panic(), drop it
there for being static and hence easy to determine without being part
of the panic message.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>
Link: https://lore.kernel.org/r/7b3998e3-1233-4e5a-89ec-d740e77eb166@suse.com


Signed-off-by: Juergen Gross <jgross@suse.com>