I got burnt one too many time by mismatches between
the pci_pool and "real" slabs... something changed in
mm/slab.c and broke a driver, so I'm going for the real
fix this time.  Having poisoning that _works_ is a huge
help in the innards of the USB host controller drivers.

This patch gets rid of some #ifdefs and makes the pci_pool
code poison memory if CONFIG_DEBUG_SLAB is set.
The functionality has always been there, but this makes
it simpler to get at.