md/md-llbitmap: prevent create failure bitmap UAF

llbitmap_create() publishes mddev->bitmap before reading the bitmap
superblock. This is needed because llbitmap_read_sb() can initialize a
new bitmap and flush it through helpers that use mddev->bitmap.

If llbitmap_read_sb() fails, the old cleanup dropped bitmap_info.mutex
and freed llbitmap before clearing mddev->bitmap. Readers such as
/proc/mdstat rely on bitmap_info.mutex to keep the bitmap pointer stable
while collecting bitmap stats, so they could observe the stale pointer
after the failed create path released the mutex.

Clear mddev->bitmap while still holding bitmap_info.mutex, then free the
failed llbitmap after dropping the mutex. This makes mutex-protected
readers see either a live bitmap or no bitmap.

Fixes: 5ab829f197 ("md/md-llbitmap: introduce new lockless bitmap")
Tested-by: Mykola Marzhan <mykola@meshstor.io>
Link: https://patch.msgid.link/20260802195038.164272-9-yukuai@kernel.org
Signed-off-by: Yu Kuai <yukuai@fygo.io>

drivers/md/md-llbitmap.c

@@ -1126,10 +1126,11 @@ static int llbitmap_create(struct mddev *mddev)
 	mutex_lock(&mddev->bitmap_info.mutex);
 	mddev->bitmap = llbitmap;
 	ret = llbitmap_read_sb(llbitmap);
+	if (ret)
+		mddev->bitmap = NULL;
 	mutex_unlock(&mddev->bitmap_info.mutex);
 	if (ret) {
 		kfree(llbitmap);
-		mddev->bitmap = NULL;
 	}
 
 	return ret;