block/blk-iocost: Split ioc_rqos_throttle()

block/blk-iocost: Split ioc_rqos_throttle()

Prepare for inlining iocg_lock() and iocg_unlock() by moving the code
between these two calls into a new function. No functionality has been
changed.

Reviewed-by: Hannes Reinecke <hare@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Cc: Tejun Heo <tj@kernel.org>
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Reviewed-by: Chaitanya Kulkarni <kch@nvidia.com>
Link: https://patch.msgid.link/a6d3ed953cef6669d23a80923bf46600733cbdae.1780682325.git.bvanassche@acm.org
Signed-off-by: Jens Axboe <axboe@kernel.dk>