lib/siphash.c: mark expected switch fall-throughs

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

This patch aims to suppress up to 18 missing-break-in-switch false
positives on some architectures.

Cc: Gustavo A. R. Silva <gustavo@embeddedor.com>
Cc: Kees Cook <keescook@chromium.org>
Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>
Reviewed-by: Jason A. Donenfeld <Jason@zx2c4.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>