powerpc: Only do ERAT invalidate on radix context switch on P9 DD1

powerpc: Only do ERAT invalidate on radix context switch on P9 DD1

BugLink: https://bugs.launchpad.net/ubuntu/+bug/1700819
From: Michael Neuling <mikey@neuling.org>

On P9 (Nimbus) DD2 and later, in radix mode, the move to the PID
register will implicitly invalidate the user space ERAT entries
and leave the kernel ones alone. Thus the only thing needed is
an isync() to synchronize this with subsequent uaccess's

Signed-off-by: Michael Neuling <mikey@neuling.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
(cherry picked from commit 74e27c6af56fe6898c3c8c451595746a992f0f0f)
Signed-off-by: Breno Leitao <leitao@debian.org>
Acked-by: Seth Forshee <seth.forshee@canonical.com>
Acked-by: Stefan Bader <stefan.bader@canonical.com>
Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@canonical.com>