UBUNTU-CVE-2026-90278: Medium severity ubuntu/linux-hwe-edge vulnerability

In the Linux kernel, the following vulnerability has been resolved: md: wait for behind writes before destroying bitmap mdstop() destroyed the bitmap before calling mddevdetach(). That made mddevdetach() skip bitmapops->waitbehindwrites(), because the bitmap was already disconnected from mddev. This was still safe for the legacy bitmap because bitmapdestroy() waits for behind writes itself. llbitmap keeps that wait in its ->waitbehindwrites() 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 mddevdetach() before mdbitmapdestroy() 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.