x86/EFI: avoid IOMMU faults on [_end,__2M_rwdata_end)

x86/EFI: avoid IOMMU faults on [_end,__2M_rwdata_end)

Commit c9a4a1c419 ("x86/layout: Correct Xen's idea of its own memory
layout") didn't go far enough with the conversion, causing IOMMU faults
when memory from that range was handed to a domain. We must not make
this memory available for allocation (the change is benign to xen.gz at
this point in time).

Note that the change to tboot_shutdown() is fixing another issue at
once: As it looks, the function so far skipped all memory below the Xen
image.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>