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

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

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>