Cleanup ext4's multi-block allocator, including adding some unit tests, as well as cleaning how we update the backup superblock after online resizes or updating the label or uuid. Optimize handling of released data blocks in ext4's commit machinery to avoid a potential lock contention on s_md_lock spinlock. Fix a number of ext4 bugs: - fix race between writepages and remount - fix racy may inline data check in dio write - add missed brelse in an error path in update_backups - fix umask handling when ACL support is disabled - fix lost EIO error when a journal commit races with a fsync of the blockdev - fix potential improper i_size when there is a crash right after an O_SYNC direct write. - check extent node for validity before potentially using what might be an invalid pointer - fix potential stale data exposure when writing to an unwritten extent and the file system is nearly out of space - fix potential accounting error around block reservations when writing partial delayed allocation writes to a bigalloc cluster - avoid memory allocation failure when tracking partial delayed allocation writes to a bigalloc cluster - fix various debugging print messages