kernel/vm: Distinguish reserved areas from non-existent areas in vm_area_for.

RESERVED_AREA_ID == -1, so previously they weren't distinguishable.
Now we return a different error code for an address that's in a
reserved area.

src/system/kernel/vm/vm.cpp

@@ -3467,6 +3467,9 @@ vm_area_for(addr_t address, bool kernel)
 				&& (area->protection & (B_READ_AREA | B_WRITE_AREA | B_CLONEABLE_AREA)) == 0)
 			return B_ERROR;
 
+		if (area->id == RESERVED_AREA_ID)
+			return EADDRINUSE;
+
 		return area->id;
 	}