ANDROID: KVM: arm64: Fix sparse __percpu warning

Force the cast to silence the warning.

We don't have a proper way to dynamically allocate memory at EL2,
and hence no proper way to dynamically allocate percpu fields.
Instead, we rely on memory donated from the host and index it by
hyp_smp_processor_id().

Reported-by: Todd Kjos <tkjos@google.com>
Signed-off-by: Fuad Tabba <tabba@google.com>
Bug: 278749606
Change-Id: I3f7efd4a41294a6696992ce0a49270c4f9468bb5