x86/32on64: properly honor add-to-physmap-batch's size

x86/32on64: properly honor add-to-physmap-batch's size

Commit 407a3c00ff ("compat/memory: fix build with old gcc") "fixed" a
build issue by switching to the use of uninitialized data. Due to
- the bounding of the uninitialized data item
- the accessed area being outside of Xen space
- arguments being properly verified by the native hypercall function
this is not a security issue.

Reported-by: Marek Marczykowski-Górecki <marmarek@invisiblethingslab.com>
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Wei Liu <wei.liu2@citrix.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
Release-acked-by: Julien Grall <julien.grall@arm.com>