xen: fix multicall debug data referencing

The recent adding of multicall debug mixed up the referencing of
the debug data. A __percpu tagged pointer can't be initialized with a
plain pointer, so use another percpu variable for the pointer and set
it on each new cpu via a function.

Fixes: 942d917c

 ("xen: make multicall debug boot time selectable")
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202407151106.5s7Mnfpz-lkp@intel.com/


Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Signed-off-by: Juergen Gross <jgross@suse.com>