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.