o Fix a race between three threads: output path,   incoming ARP packet and route request adding/removing   ARP entries. The root of the problem is that   struct llinfo_arp was accessed without any locks.   To close race we will use locking provided by   rtentry, that references this llinfo_arp:   - Make arplookup() return a locked rtentry.   - In arpresolve() hold the lock provided by     rt_check()/arplookup() until the end of function,     covering all accesses to the rtentry itself and     llinfo_arp it refers to.   - In in_arpinput() do not drop lock provided by     arplookup() during first part of the function.   - Simplify logic in the first part of in_arpinput(),     removing one level of indentation.   - In the second part of in_arpinput() hold rtentry     lock while copying address.
o Fix a race between three threads: output path,
  incoming ARP packet and route request adding/removing
  ARP entries. The root of the problem is that
  struct llinfo_arp was accessed without any locks.
  To close race we will use locking provided by
  rtentry, that references this llinfo_arp:
  - Make arplookup() return a locked rtentry.
  - In arpresolve() hold the lock provided by
    rt_check()/arplookup() until the end of function,
    covering all accesses to the rtentry itself and
    llinfo_arp it refers to.
  - In in_arpinput() do not drop lock provided by
    arplookup() during first part of the function.
  - Simplify logic in the first part of in_arpinput(),
    removing one level of indentation.
  - In the second part of in_arpinput() hold rtentry
    lock while copying address.

o Fix a condition when route entry is destroyed, while
  another thread is contested on its lock:
  - When storing a pointer to rtentry in llinfo_arp list,
    always add a reference to this rtentry, to prevent
    rtentry being destroyed via RTM_DELETE request.
  - Remove this reference when removing entry from
    llinfo_arp list.

o Further cleanup of arptimer():
  - Inline arptfree() into arptimer().
  - Use official queue(3) way to pass LIST.
  - Hold rtentry lock while reading its structure.
  - Do not check that sdl_family is AF_LINK, but
    assert this.

Reviewed by:	sam
Stress test:	http://www.holm.cc/stress/log/cons141.html
Stress test:	http://people.freebsd.org/~pho/stress/log/cons144.html

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