KVM: Check validity of offset+length of gfn_to_pfn_cache prior to activation

When activating a gfn_to_pfn_cache, verify that the offset+length is sane
and usable before marking the cache active.  Letting __kvm_gpc_refresh()
detect the problem results in a cache being marked active without setting
the GPA (or any other fields), which in turn results in KVM trying to
refresh a cache with INVALID_GPA.

Attempting to refresh a cache with INVALID_GPA isn't functionally
problematic, but it runs afoul of the sanity check that exactly one of
GPA or userspace HVA is valid, i.e. that a cache is either GPA-based or
HVA-based.

Reported-by: [email protected]
Closes: https://lore.kernel.org/all/[email protected]
Fixes: 721f5b0dda78 ("KVM: pfncache: allow a cache to be activated with a fixed (userspace) HVA")
Cc: David Woodhouse <[email protected]>
Cc: Paul Durrant <[email protected]>
Reviewed-by: Paul Durrant <[email protected]>
Reviewed-by: David Woodhouse <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Sean Christopherson <[email protected]>

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