From: Keith Owens <kaos@sgi.com>

New pdflush threads are launched on-demand by pdflush.

It turns out that on some architectures (eg, ia64) a kernel thread inherits
its parent's stack utilisation.  So after the thread-launches-a-thread
cycle has progressed sufficiently far we run out of stack space and crash.

Simple fix: convert pdflush to use kthreads.  All kthreads are parented by
keventd so there is no stack windup as a result of pdflush launching
pdflush.