ring-buffer: Use fallthrough pseudo-keyword

Replace /* fall through */ comment with pseudo-keyword macro fallthrough[1]

[1] https://www.kernel.org/doc/html/latest/process/deprecated.html?highlight=fallthrough#implicit-switch-case-fall-through

Link: https://lkml.kernel.org/r/20210511140246.18868-1-jj251510319013@gmail.com

Signed-off-by: Wei Ming Chen <jj251510319013@gmail.com>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>