lib: Fix fall-through warnings for Clang

In preparation to enable -Wimplicit-fallthrough for Clang, fix multiple
warnings by explicitly adding multiple break statements instead of
letting the code fall through to the next case, and by replacing a
number of /* fall through */ comments with the new pseudo-keyword
macro fallthrough.

Notice that Clang doesn't recognize /* Fall through */ comments as
implicit fall-through markings.

Link: https://github.com/KSPP/linux/issues/115
Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>