block: decrement block_rq_qos static key in rq_qos_del()

rq_qos_add() increments the block_rq_qos static key when a QoS
policy is attached. When a QoS policy is removed via rq_qos_del(),
we must symmetrically decrement the static key. If this removal drops
the last QoS policy from the queue (q->rq_qos becomes NULL), the
static branch can be disabled and the jump label patched to a NOP,
avoiding overhead on the hot path.

This change ensures rq_qos_add()/rq_qos_del() keep the
block_rq_qos static key balanced and prevents leaving the branch
permanently enabled after the last policy is removed.

Fixes: 033b667a823e ("block: blk-rq-qos: guard rq-qos helpers by static key")
Signed-off-by: Nilay Shroff <nilay@linux.ibm.com>
Reviewed-by: Ming Lei <ming.lei@redhat.com>
Reviewed-by: Yu Kuai <yukuai3@huawei.com>
Link: https://lore.kernel.org/r/20250814082612.500845-3-nilay@linux.ibm.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>