nbd: a BlockExport always has a BlockBackend

exp->common.blk cannot be NULL, nbd_export_delete() is only called (through
a bottom half) from blk_exp_unref() and in turn that can only happen
after blk_exp_add() has asserted exp->blk != NULL.

Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

nbd/server.c

@@ -1847,15 +1847,13 @@ static void nbd_export_delete(BlockExport *blk_exp)
     g_free(exp->description);
     exp->description = NULL;
 
-    if (exp->common.blk) {
-        if (exp->eject_notifier_blk) {
-            notifier_remove(&exp->eject_notifier);
-            blk_unref(exp->eject_notifier_blk);
-        }
-        blk_remove_aio_context_notifier(exp->common.blk, blk_aio_attached,
-                                        blk_aio_detach, exp);
-        blk_set_disable_request_queuing(exp->common.blk, false);
+    if (exp->eject_notifier_blk) {
+        notifier_remove(&exp->eject_notifier);
+        blk_unref(exp->eject_notifier_blk);
     }
+    blk_remove_aio_context_notifier(exp->common.blk, blk_aio_attached,
+                                    blk_aio_detach, exp);
+    blk_set_disable_request_queuing(exp->common.blk, false);
 
     for (i = 0; i < exp->nr_export_bitmaps; i++) {
         bdrv_dirty_bitmap_set_busy(exp->export_bitmaps[i], false);