powerpc/64s: Fix mm_cpumask memory ordering comment

The memory ordering comment no longer applies, because mm_ctx_id is
no longer used anywhere. At best always been difficult to follow.

It's better to consider the load on which the slbmte depends on, which
the MMU depends on before it can start loading TLBs, rather than a
store which may or may not have a subsequent dependency chain to the
slbmte.

So update the comment and we use the load of the mm's user context ID.
This is much more analogous the radix ordering too, which is good.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210421151733.212858-1-npiggin@gmail.com