CVE-2026-43309: md raid: fix hang when stopping arrays with metadata through dm-raid

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

md raid: fix hang when stopping arrays with metadata through dm-raid

When using device-mapper's dm-raid target, stopping a RAID array can cause the system to hang under specific conditions.

This occurs when:

- A dm-raid managed device tree is suspended from top to bottom (the top-level RAID device is suspended first, followed by its underlying metadata and data devices)

- The top-level RAID device is then removed

Removing the top-level device triggers a hang in the following sequence: the dm-raid destructor calls mdstop(), which tries to flush the write-intent bitmap by writing to the metadata sub-devices. However, these devices are already suspended, making them unable to complete the write-intent operations and causing an indefinite block.

Fix:

- Prevent bitmap flushing when mdstop() is called from dm-raid destructor context and avoid a quiescing/unquescing cycle which could also cause I/O

- Still allow write-intent bitmap flushing when called from dm-raid suspend context

This ensures that RAID array teardown can complete successfully even when the underlying devices are in a suspended state.

This second patch uses mdisrdwr() to distinguish between suspend and destructor paths as elaborated on above.