md/raid5-cache: fix null-ptr-deref for r5l_flush_stripe_to_raid()

r5l_flush_stripe_to_raid() will check if the list 'flushing_ios' is
empty, and then submit 'flush_bio', however, r5l_log_flush_endio()
is clearing the list first and then clear the bio, which will cause
null-ptr-deref:

T1: submit flush io
raid5d
 handle_active_stripes
  r5l_flush_stripe_to_raid
   // list is empty
   // add 'io_end_ios' to the list
   bio_init
   submit_bio
   // io1

T2: io1 is done
r5l_log_flush_endio
 list_splice_tail_init
 // clear the list
			T3: submit new flush io
			...
			r5l_flush_stripe_to_raid
			 // list is empty
			 // add 'io_end_ios' to the list
			 bio_init
 bio_uninit
 // clear bio->bi_blkg
			 submit_bio
			 // null-ptr-deref

Fix this problem by clearing bio before clearing the list in
r5l_log_flush_endio().

Fixes: 0dd00cba

 ("raid5-cache: fully initialize flush_bio when needed")
Reported-and-tested-by: Corey Hickey <bugfood-ml@fatooh.org>
Closes: https://lore.kernel.org/all/cddd7213-3dfd-4ab7-a3ac-edd54d74a626@fatooh.org/
Signed-off-by: Yu Kuai <yukuai3@huawei.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Song Liu <song@kernel.org>