x86/xen: Fix xen_e820_swap_entry_with_ram()

x86/xen: Fix xen_e820_swap_entry_with_ram()

When swapping a not page-aligned E820 map entry with RAM, the start
address of the modified entry is calculated wrong (the offset into the
page is subtracted instead of being added to the page address).

Fixes: be35d91c8880 ("xen: tolerate ACPI NVS memory overlapping with Xen allocated memory")
Reported-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Signed-off-by: Juergen Gross <jgross@suse.com>
Message-ID: <20260505102417.208138-1-jgross@suse.com>