md: Fix missing release of 'active_io' for flush

In the Linux kernel, the following vulnerability has been resolved: md: Fix missing release of 'active_io' for flush submit_flushes atomic_set(&mddev->flush_pending, 1); rdev_for_each_rcu(rdev, mddev) atomic_inc(&mddev->flush_pending); bi->bi_end_io = md_end_flush submit_bio(bi); /* flush io is done first */ md_end_flush if (atomic_dec_and_test(&mddev->flush_pending)) percpu_ref_put(&mddev->active_io) -> active_io is not released if (atomic_dec_and_test(&mddev->flush_pending)) -> missing release of active_io For consequence, mddev_suspend() will wait for 'active_io' to be zero forever. Fix this problem by releasing 'active_io' in submit_flushes() if 'flush_pending' is decreased to zero.