drm/i915/gem: Prevent NULL pointer dereference on missing ctx->vm

If the ctx->vm is freed before we can acquire a local reference to it,
we proceed to call i915_vm_put(NULL), which is invalid.
Reported-by: Colin Ian King <colin.king@canonical.com>
Fixes: 5dbd2b7b ("drm/i915/gem: Convert vm idr to xarray")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Colin Ian King <colin.king@canonical.com>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200123152602.1432282-1-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/gem/i915_gem_context.c

@@ -1002,13 +1002,13 @@ static int get_ppgtt(struct drm_i915_file_private *file_priv,
 	if (!rcu_access_pointer(ctx->vm))
 		return -ENODEV;
 
-	err = -ENODEV;
 	rcu_read_lock();
 	vm = context_get_vm_rcu(ctx);
-	if (vm)
-		err = xa_alloc(&file_priv->vm_xa, &id, vm,
-			       xa_limit_32b, GFP_KERNEL);
 	rcu_read_unlock();
+	if (!vm)
+		return -ENODEV;
+
+	err = xa_alloc(&file_priv->vm_xa, &id, vm, xa_limit_32b, GFP_KERNEL);
 	if (err)
 		goto err_put;