powerpc/64s/powernv: Fix memory corruption when saving SLB entries on MCE

This can be hit by an HPT guest running on an HPT host and bring down
the host, so it's quite important to fix.

Fixes: 7290f3b3

 ("powerpc/64s/powernv: machine check dump SLB contents")
Cc: stable@vger.kernel.org # v5.4+
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Acked-by: Mahesh Salgaonkar <mahesh@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20201128070728.825934-2-npiggin@gmail.com