KVM: arm64: vgic: Handle const qualifier from gic_kvm_info allocation type

In preparation for making the kmalloc family of allocators type aware,
we need to make sure that the returned type from the allocation matches
the type of the variable being assigned. (Before, the allocator would
always return "void *", which can be implicitly cast to any pointer type.)

The assigned type is "struct gic_kvm_info", but the returned type,
while matching, is const qualified. To get them exactly matching, just
use the dereferenced pointer for the sizeof().

Signed-off-by: Kees Cook <kees@kernel.org>
Link: https://patch.msgid.link/20260206223022.it.052-kees@kernel.org
Signed-off-by: Marc Zyngier <maz@kernel.org>