fix for older compiler than gcc 5 Reference: kernel's include/linux/compiler_attributes.h
commit 294f69e662d1570703e9b56e95be37a9fd3afba5 Author: Joe Perches <joe@perches.com> Date: Sat Oct 5 09:46:42 2019 -0700 compiler_attributes.h: Add 'fallthrough' pseudo keyword for switch/case use