Finally remove the kernel lock from IPv6 neighbor discovery. ND6

Finally remove the kernel lock from IPv6 neighbor discovery.  ND6
entries in rt_llinfo are protected either by exclusive netlock or
the ND6 mutex.  The performance critical lookup path in nd6_resolve()
uses shared netlock, but is not lockless.  In contrast to ARP it
grabs the mutex also in the common case.
tested by Hrvoje Popovski; with and OK kn@