net: Fix LLE lock leaks

Historically, lltable_try_set_entry_addr() would release the LLE lock
upon failure. After some refactoring, it no longer does so, but
consumers were not adjusted accordingly.

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

Reviewed by: bz, melifaro
Reported by: pho
Fixes: 0b79b007ebfc ("[lltable] Restructure nd6 code.")
MFC after: 3 days
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D34831