powerpc/64s/hash: Use POWER9 SLBIA IH=3 variant in switch_slb

POWER9 introduces SLBIA IH=3, which invalidates all SLB entries and
associated lookaside information that have a class value of 1, which
Linux assigns to user addresses. This matches what switch_slb wants,
and allows a simple fast implementation that avoids the slb_cache
complexity.

As a side-effect, the POWER5 < DD2.1 SLB invalidation workaround is
also avoided on POWER9.

Process context switching rate is improved about 2.2% for a small
process that hits the slb cache which is the best case for the current
code.

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