md/raid5: In ops_run_io, inc nr_pending before calling md_wait_for_blocked_rdev

In ops_run_io(), the call to md_wait_for_blocked_rdev will decrement
nr_pending so we lose the reference we hold on the rdev.
So atomic_inc it first to maintain the reference.

This bug was introduced by commit  73e92e51
    md/raid5.  Don't write to known bad block on doubtful devices.

which appeared in 3.0, so patch is suitable for stable kernels since
then.

Cc: stable@vger.kernel.org
Signed-off-by: Nmajianpeng <majianpeng@gmail.com>
Signed-off-by: NNeilBrown <neilb@suse.de>

drivers/md/raid5.c

@@ -606,6 +606,12 @@ static void ops_run_io(struct stripe_head *sh, struct stripe_head_state *s)
 					 * a chance*/
 					md_check_recovery(conf->mddev);
 				}
+				/*
+				 * Because md_wait_for_blocked_rdev
+				 * will dec nr_pending, we must
+				 * increment it first.
+				 */
+				atomic_inc(&rdev->nr_pending);
 				md_wait_for_blocked_rdev(rdev, conf->mddev);
 			} else {
 				/* Acknowledged bad block - skip the write */