memory: fix XSA-158 fix

memory: fix XSA-158 fix

For one the uses of domu_max_order and ptdom_max_order were swapped.

And then gcc warns about an unused result of a __must_check function
in the control part of a conditional expression when both other
expressions can be determined by the compiler to produce the same value
(see https://gcc.gnu.org/bugzilla/show_bug.cgi?id=68039), which happens
when HAS_PASSTHROUGH is undefined (i.e. for ARM on 4.4 and older).

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>