[PARISC] Use DEBUG_KERNEL to catch used-after-free __init data

Use CONFIG_DEBUG_KERNEL to catch kernel code which tries to access
__init data after it is freed. When CONFIG_DEBUG_KERNEL is not set
this also cleans up a WARN_ON at boot time. Also remove some dead
code from mm/init.c

Signed-off-by: Helge Deller <deller@parisc-linux.org>
Signed-off-by: Kyle McMartin <kyle@parisc-linux.org>