powerpc/64s: Fix stress_hpt memblock alloc alignment

The stress_hpt memblock allocation did not pass in an alignment,
which causes a stack dump in early boot (that I missed, oops).

Fixes: 6b34a099 ("powerpc/64s/hash: add stress_hpt kernel boot option to increase hash faults")
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20221216115930.2667772-2-npiggin@gmail.com