powerpc: Fix CONFIG_DEBUG_PAGEALLOC on 603/e300

So we tried to speed things up a bit using flush_hash_pages() directly
but that falls over on 603 of course meaning we fail to flush the TLB
properly and we may even end up having it corrupt memory randomly by
accessing a hash table that doesn't exist.

This removes the "optimization" by always going through flush_tlb_page()
for now at least.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>