xen: fix multicall debug feature

Initializing a percpu variable with the address of a struct tagged as
.initdata is breaking the build with CONFIG_SECTION_MISMATCH_WARN_ONLY
not set to "y".

Fix that by using an access function instead returning the .initdata
struct address if the percpu space of the struct hasn't been
allocated yet.

Fixes: 368990a7 ("xen: fix multicall debug data referencing")
Reported-by: Borislav Petkov <bp@alien8.de>
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Acked-by: "Borislav Petkov (AMD)" <bp@alien8.de>
Tested-by: "Borislav Petkov (AMD)" <bp@alien8.de>
Signed-off-by: Juergen Gross <jgross@suse.com>
Message-ID: <20250327190602.26015-1-jgross@suse.com>