powerpc/powernv/memtrace: Fix dcache flushing

Trace memory is cleared and the corresponding dcache lines
are flushed after allocation. However, this should not be
done using the PFN. This adds the missing conversion to
virtual address.

Fixes: 2ac02e5e

 ("powerpc/mm: Remove dcache flush from memory remove.")
Signed-off-by: Sandipan Das <sandipan@linux.ibm.com>
Reviewed-by: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210501160254.1179831-1-sandipan@linux.ibm.com