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>

arch/powerpc/platforms/powernv/vas-window.c

@@ -1063,15 +1063,15 @@ struct vas_window *vas_tx_win_open(int vasid, enum vas_cop_type cop,
 			rc = PTR_ERR(txwin->paste_kaddr);
 			goto free_window;
 		}
-	}
-
+	} else {
 		/*
-	 * Now that we have a send window, ensure context switch issues
+		 * A user mapping must ensure that context switch issues
 		 * CP_ABORT for this thread.
 		 */
-	rc = -EINVAL;
-	if (set_thread_uses_vas() < 0)
+		rc = set_thread_uses_vas();
+		if (rc)
 			goto free_window;
+	}
 
 	set_vinst_win(vinst, txwin);