/***************************************************************************
* This routine walks through an ASM list and marks the PDAs that have
* failed. It's called only when a disk failure causes an in-flight
* DAG to fail. The parity may consist of two components, but we want
* to use only one failedPDA pointer. Thus we set failedPDA to point
* to the first parity component, and rely on the rest of the code to
* do the right thing with this.
***************************************************************************/