cfq: Annotate fall-through in a switch statement

This patch avoids that gcc complains about fall-through when building
with W=1.

Signed-off-by: Bart Van Assche <bart.vanassche@wdc.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Danny Lin <danny@kdrag0n.dev>
Signed-off-by: Pranav Vashi <neobuddy89@gmail.com>