* Add a warning when memory encryption conversions fail. These

   operations require VMM cooperation, even in CoCo environments
   where the VMM is untrusted.  While it's _possible_ that memory
   pressure could trigger the new warning, the odds are that a
   guest would only see this from an attacking VMM.
 * Simplify page fault code by reenabling interrupts unconditionally
 * Avoid truncation issues when pfns are passed in to
   pfn_to_kaddr() with small (<64-bit) types.