md: suspend array while updating raid_disks via sysfs

In raid1_reshape(), freeze_array() is called before modifying the r1bio
memory pool (conf->r1bio_pool) and conf->raid_disks, and
unfreeze_array() is called after the update is completed.

However, freeze_array() only waits until nr_sync_pending and
(nr_pending - nr_queued) of all buckets reaches zero. When an I/O error
occurs, nr_queued is increased and the corresponding r1bio is queued to
either retry_list or bio_end_io_list. As a result, freeze_array() may
unblock before these r1bios are released.

This can lead to a situation where conf->raid_disks and the mempool have
already been updated while queued r1bios, allocated with the old
raid_disks value, are later released. Consequently, free_r1bio() may
access memory out of bounds in put_all_bios() and release r1bios of the
wrong size to the new mempool, potentially causing issues with the
mempool as well.

Since only normal I/O might increase nr_queued while an I/O error occurs,
suspending the array avoids this issue.

Note: Updating raid_disks via ioctl SET_ARRAY_INFO already suspends
the array. Therefore, we suspend the array when updating raid_disks
via sysfs to avoid this issue too.

Signed-off-by: FengWei Shih <dannyshih@synology.com>
Link: https://lore.kernel.org/linux-raid/20251226101816.4506-1-dannyshih@synology.com
Signed-off-by: Yu Kuai <yukuai@fnnas.com>

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