Kernel: Fix my result propagation bug @BenWiederhake spotted

In #3001 I was trying to fix result propagation issues, and
I actually just introduced another one. Luckily Ben spotted
it in the diff after it was in the tree, thanks Ben!

Kernel/FileSystem/FileDescription.cpp

@@ -197,7 +197,7 @@ ssize_t FileDescription::get_dir_entries(u8* buffer, ssize_t size)
     });
 
     if (result.is_error())
-        result.error();
+        return result;
 
     stream.snip();