powerpc/64s/hash: Use POWER6 SLBIA IH=1 variant in switch_slb

The SLBIA IH=1 hint will remove all non-zero SLBEs, but only
invalidate ERAT entries associated with a class value of 1, for
processors that support the hint (e.g., POWER6 and newer), which
Linux assigns to user addresses.

This prevents kernel ERAT entries from being invalidated when
context switchig (if the thread faulted in more than 8 user SLBEs).

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>