powerpc/32s: fix flush_hash_pages() on SMP

flush_hash_pages() runs with data translation off, so current
task_struct has to be accesssed using physical address.

Fixes: f7354cca ("powerpc/32: Remove CURRENT_THREAD_INFO and rename TI_CPU")
Cc: stable@vger.kernel.org # v5.1+
Reported-by: Erhard F. <erhard_f@mailbox.org>
Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>