kernel/vm: Remove areas from the areas map all at once if possible.

This way we avoid lock contention if multiple threads are trying
to delete areas at once.

Same benchmark and "before" as previous commit:
real    0m14.522s
user    0m14.194s
sys     0m4.337s

after:
real    0m13.810s
user    0m14.145s
sys     0m4.155s

src/system/kernel/vm/vm.cpp

@@ -2570,7 +2570,8 @@ delete_area(VMAddressSpace* addressSpace, VMArea* area,
 {
 	ASSERT(!area->IsWired());
 
-	VMAreas::Remove(area);
+	if (area->id >= 0)
+		VMAreas::Remove(area);
 
 	// At this point the area is removed from the global hash table, but
 	// still exists in the area list.
@@ -3992,6 +3993,17 @@ vm_delete_areas(struct VMAddressSpace* addressSpace, bool deletingAddressSpace)
 	// remove all reserved areas in this address space
 	addressSpace->UnreserveAllAddressRanges(0);
 
+	// remove all areas from the areas map at once (to avoid lock contention)
+	VMAreas::WriteLock();
+	{
+		VMAddressSpace::AreaIterator it = addressSpace->GetAreaIterator();
+		while (VMArea* area = it.Next()) {
+			VMAreas::Remove(area);
+			area->id = INT32_MIN;
+		}
+	}
+	VMAreas::WriteUnlock();
+
 	// delete all the areas in this address space
 	while (VMArea* area = addressSpace->FirstArea()) {
 		ASSERT(!area->IsWired());