Add AsFd impl for Link type

This change introduces an AsFd implementation for the Link type, similar
to what we already have for Map and Program.

Signed-off-by: Daniel Müller <[email protected]>

Author: d-e-s-o

libbpf-rs/src/link.rs

@@ -1,4 +1,6 @@
 use std::fmt::Debug;
+use std::os::unix::io::AsFd;
+use std::os::unix::io::BorrowedFd;
 use std::path::Path;
 use std::path::PathBuf;
 use std::ptr::NonNull;
@@ -108,6 +110,17 @@ impl Link {
     }
 }
 
+impl AsFd for Link {
+    #[inline]
+    fn as_fd(&self) -> BorrowedFd<'_> {
+        let fd = unsafe { libbpf_sys::bpf_link__fd(self.ptr.as_ptr()) };
+        // SAFETY: `bpf_link__fd` always returns a valid fd and the underlying
+        //         libbpf object is not destroyed until the object is dropped,
+        //         which means the fd remains valid as well.
+        unsafe { BorrowedFd::borrow_raw(fd) }
+    }
+}
+
 impl Drop for Link {
     fn drop(&mut self) {
         let _ = unsafe { libbpf_sys::bpf_link__destroy(self.ptr.as_ptr()) };