Fix multiple issues pointed out by the new compiler

kernel/src/syscall/getdents64.rs

@@ -104,7 +104,7 @@ impl<'a, T: DirentSerializer> DirentBufferReader<'a, T> {
     }
 }
 
-impl<'a, T: DirentSerializer> DirentVisitor for DirentBufferReader<'a, T> {
+impl<T: DirentSerializer> DirentVisitor for DirentBufferReader<'_, T> {
     fn visit(&mut self, name: &str, ino: u64, type_: InodeType, offset: usize) -> Result<()> {
         let dirent_serializer = T::new(ino, offset as u64, type_, CString::new(name)?);
         if self.read_len >= self.buffer.len() {

kernel/src/syscall/mmap.rs

@@ -218,6 +218,7 @@ impl MMapOptions {
         self.typ
     }
 
+    #[allow(unused)]
     pub fn flags(&self) -> MMapFlags {
         self.flags
     }

kernel/src/syscall/poll.rs

@@ -158,7 +158,7 @@ impl From<c_pollfd> for PollFd {
 
 impl From<PollFd> for c_pollfd {
     fn from(raw: PollFd) -> Self {
-        let fd = if let Some(fd) = raw.fd() { fd } else { -1 };
+        let fd = raw.fd().unwrap_or(-1);
         let events = raw.events().bits() as i16;
         let revents = raw.revents().get().bits() as i16;
         Self {