md: wait for behind writes before destroying bitmap

In the Linux kernel, the following vulnerability has been resolved: md: wait for behind writes before destroying bitmap __md_stop() destroyed the bitmap before calling mddev_detach(). That made mddev_detach() skip bitmap_ops->wait_behind_writes(), because the bitmap was already disconnected from mddev. This was still safe for the legacy bitmap because bitmap_destroy() waits for behind writes itself. llbitmap keeps that wait in its ->wait_behind_writes() operation instead, while ->destroy() tears down the llbitmap storage. With the old ordering, RAID1 behind-write completions could still run after llbitmap storage had been freed. Call mddev_detach() before md_bitmap_destroy() so the common detach path can wait for behind writes while the bitmap is still alive. Only destroy the bitmap after those users are gone.