x86/mm: Prevent 32bit PV guests using out-of-range linear addresses

x86/mm: Prevent 32bit PV guests using out-of-range linear addresses

The grant ABI uses 64 bit values, and allows a PV guest to specify linear
addresses.  There is nothing interesting a 32bit PV guest can reference which
will pass an __addr_ok() check (and therefore succeed), but we should still
explicitly check and reject such an attempt.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>