raid5: Use slow_path to release stripe when mddev->thread is null

When release_stripe() is called in grow_one_stripe(), the
mddev->thread is null. So it will omit one wakeup this thread to
release stripe.
For this condition, use slow_path to release stripe.

Bug was introduced in 3.12

Cc: stable@vger.kernel.org (3.12+)
Fixes: 773ca82fa1ee58dd1bf88b
Signed-off-by: Jianpeng Ma <majianpeng@gmail.com>
Signed-off-by: NeilBrown <neilb@suse.de>