Smack: Mark expected switch fall-through

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

Notice that in this particular case, I replaced "No break" with a
proper "Fall through" annotation, which is what GCC is expecting
to find.

Warning level 2 was used: -Wimplicit-fallthrough=2

Addresses-Coverity-ID: 115051 ("Missing break in switch")
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: Casey Schaufler <casey@schaufler-ca.com>