fs: Ensure fs_dupfd2 always return fd2 in the sucessful path

it's wrong to return file_dup2 directly since file_dup2 never return file handle

Signed-off-by: Xiang Xiao <xiaoxiang@xiaomi.com>
Change-Id: I943537849c75d83b3d646a6a22f035187d9fd521

fs/vfs/fs_dupfd2.c

@@ -85,5 +85,11 @@ int fs_dupfd2(int fd1, int fd2)
 
   /* Perform the dup2 operation */
 
-  return file_dup2(filep1, filep2);
+  ret = file_dup2(filep1, filep2);
+  if (ret < 0)
+    {
+      return ret;
+    }
+
+  return fd2;
 }