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.