netlink: fix potential llentry lock leak in newneigh handler

The netlink newneigh handler has the potential to leak the lock on
llentry objects in the kernel. This patch reconciles several paths
through the newneigh handler that could result in a lock leak.

MFC after: 1 week
Reviewed by: markj, kp
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D42307

(cherry picked from commit ae2ca32781a90abe987e128ca167ab400a87f369)