Use libc::syscall instead of libc::copy_file_range

It seems that many Linux distros don't include copy_file_range in their
C libraries.

src/fcntl.rs

@@ -645,16 +645,34 @@ pub fn copy_file_range<Fd1: AsFd, Fd2: AsFd>(
         .map(|offset| offset as *mut type_of_off)
         .unwrap_or(ptr::null_mut());
 
-    let ret = unsafe {
-        libc::copy_file_range(
-            fd_in.as_fd().as_raw_fd(),
-            off_in,
-            fd_out.as_fd().as_raw_fd(),
-            off_out,
-            len,
-            0,
-        )
-    };
+    cfg_if::cfg_if! {
+        if #[cfg(target_os = "freebsd")] {
+            let ret = unsafe {
+                libc::copy_file_range(
+                    fd_in.as_fd().as_raw_fd(),
+                    off_in,
+                    fd_out.as_fd().as_raw_fd(),
+                    off_out,
+                    len,
+                    0,
+                )
+            };
+        } else {
+            // May Linux distros still don't include copy_file_range in their
+            // libc implementations, so we need to make a direct syscall.
+            let ret = unsafe {
+                libc::syscall(
+                    libc::SYS_copy_file_range,
+                    fd_in,
+                    off_in,
+                    fd_out.as_fd().as_raw_fd(),
+                    off_out,
+                    len,
+                    0,
+                )
+            };
+        }
+    }
     Errno::result(ret).map(|r| r as usize)
 }