Clean up clippy warnings about if-else blocks

This de-duplicates and (hopefully) simplifies a few if-else blocks. The
others were given an exception because I thought they were more readable
as is. I've verified that these changes don't result in larger binaries.

src/iface/ethernet.rs

@@ -1026,12 +1026,9 @@ impl<'b, 'c, 'e> InterfaceInner<'b, 'c, 'e> {
            !self.has_multicast_group(ipv4_repr.dst_addr) {
             // Ignore IP packets not directed at us, or broadcast, or any of the multicast groups.
             // If AnyIP is enabled, also check if the packet is routed locally.
-            if !self.any_ip {
-                return Ok(None);
-            } else if match self.routes.lookup(&IpAddress::Ipv4(ipv4_repr.dst_addr), timestamp) {
-                Some(router_addr) => !self.has_ip_addr(router_addr),
-                None => true,
-            } {
+            if !self.any_ip ||
+                    self.routes.lookup(&IpAddress::Ipv4(ipv4_repr.dst_addr), timestamp)
+                        .map_or(true, |router_addr| !self.has_ip_addr(router_addr)) {
                 return Ok(None);
             }
         }
@@ -1188,10 +1185,9 @@ impl<'b, 'c, 'e> InterfaceInner<'b, 'c, 'e> {
                 let ip_addr = ip_repr.src_addr.into();
                 match lladdr {
                     Some(lladdr) if lladdr.is_unicast() && target_addr.is_unicast() => {
-                        if flags.contains(NdiscNeighborFlags::OVERRIDE) {
+                        if flags.contains(NdiscNeighborFlags::OVERRIDE) ||
+                                !self.neighbor_cache.lookup(&ip_addr, timestamp).found() {
                             self.neighbor_cache.fill(ip_addr, lladdr, timestamp)
-                        } else if !self.neighbor_cache.lookup(&ip_addr, timestamp).found() {
-                                self.neighbor_cache.fill(ip_addr, lladdr, timestamp)
                         }
                     },
                     _ => (),

src/lib.rs

@@ -87,8 +87,6 @@
                feature = "socket-tcp")))]
 compile_error!("at least one socket needs to be enabled"); */
 
-// FIXME(dlrobertson): clippy fails with this lint
-#![allow(clippy::if_same_then_else)]
 #![allow(clippy::match_like_matches_macro)]
 #![allow(clippy::redundant_field_names)]
 #![allow(clippy::identity_op)]

src/socket/tcp.rs

@@ -1905,6 +1905,7 @@ impl<'a> TcpSocket<'a> {
         Ok(())
     }
 
+    #[allow(clippy::if_same_then_else)]
     pub(crate) fn poll_at(&self) -> PollAt {
         // The logic here mirrors the beginning of dispatch() closely.
         if !self.remote_endpoint.is_specified() {

src/wire/arp.rs

@@ -86,6 +86,7 @@ impl<T: AsRef<[u8]>> Packet<T> {
     ///
     /// [set_hardware_len]: #method.set_hardware_len
     /// [set_protocol_len]: #method.set_protocol_len
+    #[allow(clippy::if_same_then_else)]
     pub fn check_len(&self) -> Result<()> {
         let len = self.buffer.as_ref().len();
         if len < field::OPER.end {

src/wire/ipv4.rs

@@ -271,6 +271,7 @@ impl<T: AsRef<[u8]>> Packet<T> {
     ///
     /// [set_header_len]: #method.set_header_len
     /// [set_total_len]: #method.set_total_len
+    #[allow(clippy::if_same_then_else)]
     pub fn check_len(&self) -> Result<()> {
         let len = self.buffer.as_ref().len();
         if len < field::DST_ADDR.end {