Rework LLE code locking:
* struct llentry is now basically split into 2 pieces:
  all fields within 64 bytes (amd64) are now protected by both
  ifdata lock AND lle lock, e.g. you require both locks to be held
  exclusively for modification. All data necessary for fast path
  operations is kept here. Some fields were added:
  - r_l3addr - makes lookup key liev within first 64 bytes.
  - r_flags - flags, containing pre-compiled decision whether given
    lle contains usable data or not. Current the only flag is RLLE_VALID.
  - r_len - prepend data len, currently unused
  - r_kick - used to provide feedback to control plane (see below).
  All other fields are protected by lle lock.
* Add simple state machine for ARP to handle "about to expire" case:
  Current model (for the fast path) is the following:
  - rlock afdata
  - find / rlock rte
  - runlock afdata
  - see if "expire time" is approaching
    (time_uptime + la->la_preempt > la->la_expire)
  - if true, call arprequest() and decrease la_preempt
  - store MAC and runlock rte
  New model (data plane):
  - rlock afdata
  - find rte
  - check if it can be used using r_* fields only
  - if true, store MAC
  - if r_kick field != 0 set it to 0.
  - runlock afdata
  New mode (control plane):
  - schedule arptimer to be called in (V_arpt_keep - V_arp_maxtries)
    seconds instead of V_arpt_keep.
  - on first timer invocation change state from ARP_LLINFO_REACHABLE
    to ARP_LLINFO_VERIFY, sets r_kick to 1 and shedules next call in
    V_arpt_rexmit (default to 1 sec).
  - on subsequent timer invocations in ARP_LLINFO_VERIFY state, checks
    for r_kick value: reschedule if not changed, and send arprequest()
    if set to zero (e.g. entry was used).
* Convert IPv4 path to use new single-lock approach. IPv6 bits to follow.
* Slow down in_arpinput(): now valid reply will (in most cases) require
  acquiring afdata WLOCK twice. This is requirement for storing changed
  lle data. This change will be slightly optimized in future.
* Provide explicit hash link/unlink functions for both ipv4/ipv6 code.
  This will probably be moved to generic lle code once we have per-AF
  hashing callback inside lltable.
* Perform lle unlink on deletion immediately instead of delaying it to
  the timer routine.
* Make r244183 more explicit: use new LLE_CALLOUTREF flag to indicate the
  presence of lle reference used for safe callout calls.