[$] Ext4 data corruption hits the stable kernels

The kernel's stable-update process is intended to produce kernels that are,
well, stable; when that promise is lived up to, users can update to newer
stable updates without fear. By any account, a bug that corrupts data on
ext4 filesystems constitutes a failure to hold to that promise. As is so
often the case, this problem is the result of a chain of failures in a
system that works well most of the time.