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

In the Linux kernel, the following vulnerability has been resolved:

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().