[rust-netstack] Fix warnings, remove allow(warnings) annotation

- Fix all non-unused warnings
- Remove #![allow(warnings)] from crate root
- Replace with #![allow(unused)]

Test: Existing unit tests pass
Change-Id: Ib7346e887db9190aa1841328c53f00c22b877491
diff --git a/bin/recovery_netstack/core/src/device/ethernet.rs b/bin/recovery_netstack/core/src/device/ethernet.rs
index 3ba48d1..24f702f 100644
--- a/bin/recovery_netstack/core/src/device/ethernet.rs
+++ b/bin/recovery_netstack/core/src/device/ethernet.rs
@@ -165,19 +165,19 @@
 
 /// An extension trait adding IP-related functionality to `Ipv4` and `Ipv6`.
 trait EthernetIpExt: Ip {
-    const EtherType: EtherType;
+    const ETHER_TYPE: EtherType;
 }
 
 impl<I: Ip> EthernetIpExt for I {
-    default const EtherType: EtherType = EtherType::Ipv4;
+    default const ETHER_TYPE: EtherType = EtherType::Ipv4;
 }
 
 impl EthernetIpExt for Ipv4 {
-    const EtherType: EtherType = EtherType::Ipv4;
+    const ETHER_TYPE: EtherType = EtherType::Ipv4;
 }
 
 impl EthernetIpExt for Ipv6 {
-    const EtherType: EtherType = EtherType::Ipv6;
+    const ETHER_TYPE: EtherType = EtherType::Ipv6;
 }
 
 /// Send an IP packet in an Ethernet frame.
@@ -219,7 +219,7 @@
             .encapsulate(EthernetFrameSerializer::new(
                 src_mac,
                 dst_mac,
-                A::Version::EtherType,
+                A::Version::ETHER_TYPE,
             ))
             .serialize_outer();
         ctx.dispatcher()
diff --git a/bin/recovery_netstack/core/src/lib.rs b/bin/recovery_netstack/core/src/lib.rs
index 3e0b214..893978e 100644
--- a/bin/recovery_netstack/core/src/lib.rs
+++ b/bin/recovery_netstack/core/src/lib.rs
@@ -4,7 +4,6 @@
 
 //! A networking stack.
 
-
 #![feature(async_await, await_macro)]
 #![feature(const_fn)]
 #![feature(iterator_find_map)]
@@ -22,8 +21,10 @@
 #![deny(safe_packed_borrows)]
 #![deny(missing_docs)]
 #![deny(unreachable_patterns)]
-// FIXME(joshlf)
-#![allow(warnings)]
+
+// TODO(joshlf): Remove this once all of the elements in the crate are actually
+// used.
+#![allow(unused)]
 
 #[macro_use]
 mod macros;
diff --git a/bin/recovery_netstack/core/src/wire/icmp/icmpv6.rs b/bin/recovery_netstack/core/src/wire/icmp/icmpv6.rs
index 0a686ff..5bb7919 100644
--- a/bin/recovery_netstack/core/src/wire/icmp/icmpv6.rs
+++ b/bin/recovery_netstack/core/src/wire/icmp/icmpv6.rs
@@ -132,7 +132,7 @@
 #[derive(Copy, Clone)]
 #[repr(C, packed)]
 pub struct Icmpv6PacketTooBig {
-    MTU: [u8; 4],
+    mtu: [u8; 4],
 }
 
 impl_from_bytes_as_bytes_unaligned!(Icmpv6PacketTooBig);