md/raid5: use stripe state snapshot in break_stripe_batch_list()

md/raid5: use stripe state snapshot in break_stripe_batch_list()

The patch just suppress KCSAN noise. No functional change.

RAID-5 can group multi full-stripe-write aka stripe_head into a
batch aka batch_list, with one head_sh leading them. Call
break_stripe_batch_list() when the batch is finished, or,
a stripe has to be dropped out of the batch.

break_stripe_batch_list() reads stripe state several times while
request paths can update thost state words concurrently with
lockless bitops, which reported by KCSAN.

Use a snapshot to guarantees that the value used for
warning, copying, and handle checks is internally consistent
at current read moment.

KCSAN report:
==============================================
BUG: KCSAN: data-race in __add_stripe_bio / break_stripe_batch_list

write (marked) to 0xffff8e89d4f0b988 of 8 bytes by task 4323 on cpu 3:
  __add_stripe_bio+0x35e/0x400
  raid5_make_request+0x6ac/0x2930
  md_handle_request+0x4a2/0xa40
  md_submit_bio+0x109/0x1a0
  __submit_bio+0x2ec/0x390
  submit_bio_noacct_nocheck+0x457/0x710
  submit_bio_noacct+0x2a7/0xc20
  submit_bio+0x56/0x250
  blkdev_direct_IO+0x54c/0xda0
  blkdev_write_iter+0x38f/0x570
  aio_write+0x22b/0x490
  io_submit_one+0xa51/0xf70

read to 0xffff8e89d4f0b988 of 8 bytes by task 4290 on cpu 4:
  break_stripe_batch_list+0x3ce/0x480
  handle_stripe_clean_event+0x720/0x9b0
  handle_stripe+0x32fb/0x4500
  handle_active_stripes.isra.0+0x6e0/0xa50
  raid5d+0x7e0/0xba0

Signed-off-by: Chen Cheng <chencheng@fnnas.com>
Link: https://patch.msgid.link/20260618134748.1168360-1-chencheng@fnnas.com
Signed-off-by: Yu Kuai <yukuai@fygo.io>