[POWERPC] Enable SLUB in *_defconfig

When checking out the new NO_HZ support in powerpc, I noticed we never
slept for more than 2 seconds.  It turns out SLAB has a 2 second per cpu
timer that causes this.

After switching to SLUB I see some nice 4 second sleeps which is the
limit on this POWER6 box (the decrementer ticks at 512MHz):

slept 4.19 sec
slept 4.19 sec
slept 4.19 sec
slept 4.19 sec
slept 3.96 sec
slept 3.80 sec
slept 2.99 sec

Since SLUB is now the default and some powerpc defconfigs already enable
it, lets enable SLUB across the board for consistency.  While doing this
I also noticed that the maple defconfig has SLAB debugging enabled which
is sure to make your box nice and slow.  Fix that too.

Signed-off-by: Anton Blanchard <anton@samba.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>