Historically, lltable_try_set_entry_addr() would release the LLE lock
upon failure. After some refactoring, it no longer does so, but now
consumers can leak the LLE lock as a result.

Also fix a leak that can occur if lltable_calc_llheader() fails in the
ARP code, but I suspect that this failure can only occur due to a code
bug.

Reported by: pho
Fixes: 0b79b007ebfc ("[lltable] Restructure nd6 code.")