Kernel: Make PhysicalRegion::return_page() do arithmetic instead of loop

Most of the time, we will be freeing physical pages within the
full-sized zones. We can do some simple math to find the right zone
immediately instead of looping through the zones, checking each one.

We still do loop through the slack/remainder zones at the end.
There's probably an even nicer way to solve this, but this is already a
nice improvement. :^)

Kernel/Memory/PhysicalRegion.h

@@ -41,6 +41,9 @@ public:
 private:
     PhysicalRegion(PhysicalAddress lower, PhysicalAddress upper);
 
+    static constexpr size_t large_zone_size = 16 * MiB;
+    static constexpr size_t small_zone_size = 1 * MiB;
+
     NonnullOwnPtrVector<PhysicalZone> m_zones;
 
     PhysicalZone::List m_usable_zones;