Pull up following revision(s) (requested by ozaki-r in ticket #686):

sys/netinet/if_arp.c: revision 1.271
sys/netinet6/nd6_nbr.c: revision 1.151,1.152

Avoid passing NULL to nd6_dad_duplicated
Fix PR kern/53075

Fix a race condition on DAD destructions (again)

The previous fix to DAD timers was wrong; it avoided a use-after-free but
instead introduced a memory leak.  The destruction method had delegated
a destruction of a DAD timer to the timer itself and told that by setting NULL
to dp->dad_ifa.  However, the previous fix made DAD timers do nothing on
the sign.

Fixing the issue with using callout_stop isn't easy.  One approach is to have
a refcount on dp but it introduces extra complexity that we want to avoid.
The new fix falls back to using callout_halt, which was abandoned because of
softnet_lock.  Fortunately now the network stack is protected by KERNEL_LOCK
so we can remove softnet_lock from DAD timers (callout) and use callout_halt
safely.

(martin)