powerpc/64s/radix: occasionally attempt to trim mm_cpumask

A single-threaded process that is flushing its own address space is
so far the only case where the mm_cpumask is attempted to be trimmed.
This patch expands that to flush in other situations, multi-threaded
processes and external sources. For now it's a relatively simple
occasional trim attempt. The main aim is to add the mechanism,
tweaking and tuning can come with more data.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20201217134731.488135-6-npiggin@gmail.com