When CONFIG_BLK_DEV_THROTTLING is enabled, though we may not set
block cgroup's blk-throttle bps or iops limits, every bio still
enters blk_throtl_bio() firstly, then this bug will result in the
corresponding blkcg_gq's refcnt will increase by 1 for every bio.
atomit_t is an 'int' type, and if usr continually issues batches
of bios, this refcnt will overflow, which will trigger WARNING in
blkg_get() or blkg_put().

Fixes: 4667e926 ("alios: blk-throttle: fix tg NULL pointer dereference")
Reviewed-by: NJoseph Qi <joseph.qi@linux.alibaba.com>
Signed-off-by: NXiaoguang Wang <xiaoguang.wang@linux.alibaba.com>
Signed-off-by: NJoseph Qi <joseph.qi@linux.alibaba.com>