KVM: Explicitly disallow activatating a gfn_to_pfn_cache with INVALID_GPA

Explicit disallow activating a gfn_to_pfn_cache with an error gpa, i.e.
INVALID_GPA, to ensure that KVM doesn't mistake a GPA-based cache for an
HVA-based cache (KVM uses INVALID_GPA as a magic value to differentiate
between GPA-based and HVA-based caches).

WARN if KVM attempts to activate a cache with INVALID_GPA, purely so that
new caches need to at least consider what to do with a "bad" GPA, as all
existing usage of kvm_gpc_activate() guarantees gpa != INVALID_GPA.  I.e.
removing the WARN in the future is completely reasonable if doing so would
yield cleaner/better code overall.

Reviewed-by: David Woodhouse <dwmw@amazon.co.uk>
Reviewed-by: Paul Durrant <paul@xen.org>
Link: https://lore.kernel.org/r/20240320001542.3203871-4-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

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