powerpc/config: Disable SLAB_DEBUG_ON in skiroot

In 5.10 commit 5e84dd54 ("powerpc/configs/skiroot: Enable some more
hardening options") set SLUB_DEBUG_ON.

When 5.14 came around, commit 79270291 ("slub: force on
no_hash_pointers when slub_debug is enabled") print all the
pointers when SLUB_DEBUG_ON is set. This was fine, but in 5.12 commit
5ead723a ("lib/vsprintf: no_hash_pointers prints all addresses as
unhashed") added the warning at boot.

Disable SLAB_DEBUG_ON as we don't want the nasty warning. We have
CONFIG_EXPERT so SLAB_DEBUG is enabled. We do lose the settings in
DEBUG_DEFAULT_FLAGS, but it's not clear that these should have been
always-on anyway.

Signed-off-by: Joel Stanley <joel@jms.id.au>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20230705023056.16273-1-joel@jms.id.au