build: #[allow(clippy::significant_drop_in_scrutinee) in some crates

This check is new in the beta version of clippy and exists to avoid
potential deadlocks by highlighting when the test in an if or for loop
is something that holds a lock. In many cases we would need to make
significant refactorings to be able to pass this check so disable in the
affected crates.

Signed-off-by: Rob Bradford <robert.bradford@intel.com>

vhost_user_net/src/lib.rs

@@ -6,6 +6,8 @@
 //
 // SPDX-License-Identifier: (Apache-2.0 AND BSD-3-Clause)
 
+#![allow(clippy::significant_drop_in_scrutinee)]
+
 use libc::{self, EFD_NONBLOCK};
 use log::*;
 use net_util::{