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

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

commit a1ee28117077c3bf24e5ab6324c835eaab629c45 upstream.

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: 7290f3b3d3e6 ("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
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>