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