x86/mm: Reduce debug overhead of __virt_to_maddr()

x86/mm: Reduce debug overhead of __virt_to_maddr()

__virt_to_maddr() is used very frequently, but has a large footprint due to
its assertions and comparasons.

Rearange its logic to drop one assertion entirely, encoding its check in a
second assertion (with no additional branch, and the comparason performed with
a 32bit immediate rather than requiring a movabs).

Bloat-o-meter net report is:
  add/remove: 0/0 grow/shrink: 1/72 up/down: 3/-2169 (-2166)

along with a reduction of 32 assertion frames (895 down to 861)

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