md: fix resync softlockup when bitmap size is less than array size

Is is reported that for dm-raid10, lvextend + lvchange --syncaction will
trigger following softlockup:

kernel:watchdog: BUG: soft lockup - CPU#3 stuck for 26s! [mdX_resync:6976]
CPU: 7 PID: 3588 Comm: mdX_resync Kdump: loaded Not tainted 6.9.0-rc4-next-20240419 #1
RIP: 0010:_raw_spin_unlock_irq+0x13/0x30
Call Trace:
 <TASK>
 md_bitmap_start_sync+0x6b/0xf0
 raid10_sync_request+0x25c/0x1b40 [raid10]
 md_do_sync+0x64b/0x1020
 md_thread+0xa7/0x170
 kthread+0xcf/0x100
 ret_from_fork+0x30/0x50
 ret_from_fork_asm+0x1a/0x30

And the detailed process is as follows:

md_do_sync
 j = mddev->resync_min
 while (j < max_sectors)
  sectors = raid10_sync_request(mddev, j, &skipped)
   if (!md_bitmap_start_sync(..., &sync_blocks))
    // md_bitmap_start_sync set sync_blocks to 0
    return sync_blocks + sectors_skippe;
  // sectors = 0;
  j += sectors;
  // j never change

Root cause is that commit 301867b1 ("md/raid10: check
slab-out-of-bounds in md_bitmap_get_counter") return early from
md_bitmap_get_counter(), without setting returned blocks.

Fix this problem by always set returned blocks from
md_bitmap_get_counter"(), as it used to be.

Noted that this patch just fix the softlockup problem in kernel, the
case that bitmap size doesn't match array size still need to be fixed.

Fixes: 301867b1 ("md/raid10: check slab-out-of-bounds in md_bitmap_get_counter")
Reported-and-tested-by: Nigel Croxon <ncroxon@redhat.com>
Closes: https://lore.kernel.org/all/71ba5272-ab07-43ba-8232-d2da642acb4e@redhat.com/


Signed-off-by: Yu Kuai <yukuai3@huawei.com>
Link: https://lore.kernel.org/r/20240422065824.2516-1-yukuai1@huaweicloud.com


Signed-off-by: Song Liu <song@kernel.org>