We need to add memory barrier so that the page table walk doesn't happen
before the cpumask is set and made visible to the other cpus. We need
to use a sync here instead of lwsync because lwsync is not sufficient for
store/load ordering.

We also need to add an if (mm) check so that we do the right thing when called
with a kernel context. For kernel context, we have mm = NULL. W.r.t kernel
address we can skip setting the mm cpumask.

Fixes: 0f4bc093 ("powerpc/mm/cxl: Add the fault handling cpu to mm cpumask")
Cc: Andrew Donnellan <andrew.donnellan@au1.ibm.com>
Reported-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
Acked-by: Andrew Donnellan <andrew.donnellan@au1.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>