Do more fine-grained locking in lltable code: lltable_create_lle()
  does actual new lle creation without extensive locking and existing
  lle search.
Move lle updating code from gigantic in_arpinput() to arp_update_llle()
  and some other functions.

IPv6 changes to follow.
Put bandaid on arptimer-derived crashed for 'arp -da' deleted records.
The proper fix will arrive after convering lltable 'delete' method.
* Add llt_clear_entry() callback which is able to do all lle
   cleanup including unlinking/freeing
* Relax locking in lltable_prefix_free_af/lltable_free
* Do not pass @llt to lle free callback: it is always NULL now.
* Unify arptimer/nd6_llinfo_timer: explicitly unlock lle avoiding
   unlock/lock sequinces
* Do not pass unlocked lle to nd6_ns_output(): add nd6_llinfo_get_holdsrc()
   to retrieve preferred source address from lle hold queue and pass it
   instead of lle.
* Finally, make nd6_create() create and return unlocked lle
* Separate defrtr handling code from nd6_free():
   use nd6_check_del_defrtr() to check if we need to keep entry instead of
    performing GC,
   use nd6_check_recalc_defrtr() to perform actual recalc on lle removal.
* Move isRouter handling from nd6_cache_lladdr() to separate
   nd6_check_router()
* Add initial code to maintain lle runtime flags in sync.
Do not enforce particular lle storage scheme:
* move lltable allocation to per-domain callbacks.
* make llentry_link/unlink functions overridable llt methods.
* make hash table traversal another overridable llt method.
* Retire abstract llentry_free() in favor of lltable_drop_entry_queue()
   and explicit calls to RTENTRY_FREE_LOCKED()
* Use lltable_prefix_free() in arp_ifscrub to be consistent with nd6.
* Rename <lltable_|llt>_delete function to _delete_addr() to note that
   this function is used to external callers. Make this function maintain
   its own locking.
* Use lookup/unlink/clear call chain from internal callers instead of
    delete_addr.
* Fix LLE_DELETED flag handling