In preparation to enabling -Wimplicit-fallthrough, mark switch cases
where we are expecting to fall through.

In cases where a "drop through" comment was already in place, I replaced
it  with a proper "fall through" comment, which is what GCC is expecting
to find.
Signed-off-by: Gustavo A. R. Silva <garsilva@embeddedor.com>
Signed-off-by: Tejun Heo <tj@kernel.org>