KVM: SVM: Add a helper to convert a SME-aware PA back to a struct page

KVM: SVM: Add a helper to convert a SME-aware PA back to a struct page

Add __sme_pa_to_page() to pair with __sme_page_pa() and use it to replace
open coded equivalents, including for "iopm_base", which previously
avoided having to do __sme_clr() by storing the raw PA in the global
variable.

Opportunistically convert __sme_page_pa() to a helper to provide type
safety.

No functional change intended.

Link: https://lore.kernel.org/r/20240802204511.352017-2-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>