powerpc/64s/radix: combine final TLB flush and lazy tlb mm shootdown IPIs

This performs lazy tlb mm shootdown when doing the exit TLB flush when
all mm users go away and user mappings are removed, which avoids having
to do the lazy tlb mm shootdown IPIs on the final mmput when all kernel
references disappear.

powerpc/64s uses a broadcast TLBIE for the exit TLB flush if remote CPUs
need to be invalidated (unless TLBIE is disabled), so this doesn't
necessarily save IPIs but it does avoid a broadcast TLBIE which is quite
expensive.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
[mpe: Squash in preempt_disable/enable() fix from Nick]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20230524060821.148015-5-npiggin@gmail.com