bcache: Annotate switch fall-through

This patch avoids that building with W=1 triggers complaints about
switch fall-throughs.

Signed-off-by: Bart Van Assche <bart.vanassche@wdc.com>
Reviewed-by: Michael Lyle <mlyle@lyle.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>