- In nd6_rtrequest(), when caching an rtentry, don't forget   to add a reference to it; otherwise, we could later access   a freed memory.  This is believed to fix panics some users   were observing when running route6d(8), and is similar to   the fix in sys/netinet/if_ether.c,v 1.139 by glebius@.
- In nd6_rtrequest(), when caching an rtentry, don't forget
  to add a reference to it; otherwise, we could later access
  a freed memory.  This is believed to fix panics some users
  were observing when running route6d(8), and is similar to
  the fix in sys/netinet/if_ether.c,v 1.139 by glebius@.

PR:		kern/93910, kern/105437
Testing by:	Wojciech Puchar (still ongoing)

- Add rtentry locking to nd6_output() similar to rt_check().

MFC after:	4 days

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