powerpc/vas: Don't set uses_vas for kernel windows

cp_abort is only required for user windows, because kernel context
must not be preempted between a copy/paste pair.

Without this patch, the init task gets used_vas set when it runs the
nx842_powernv_init initcall, which opens windows for kernel usage.

used_vas is then never cleared anywhere, so it gets propagated into
all other tasks. It's a property of the address space, so it should
really be cleared when a new mm is created (or in dup_mmap if the
mmaps are marked as VM_DONTCOPY). For now we seem to have no such
driver, so leave that for another patch.

Fixes: 6c8e6bb2

 ("powerpc/vas: Add support for user receive window")
Cc: stable@vger.kernel.org # v4.15+
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Reviewed-by: Sukadev Bhattiprolu <sukadev@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>