[$] Block-layer error injection

Storage code has to cope with hardware that fails in inconvenient
ways, but coaxing a healthy disk into producing those failures on
demand, for testing, is usually not possible. The kernel
provides several ways to inject block-layer I/O errors, but none of those can select the
operation to fail, pick the status code to return, or target a disk
directly without employing a stacked device on top. Use of a stacked device means
the test runs against the mapper device, not the disk it was meant to
exercise. A patch
series from Christoph Hellwig adds a configurable error-injection
interface that does all three things that the current error-injection code
lacks, controlled by a per-disk debugfs
file.