MFC r210703:

MFC r210703:

  Document the mandatory argument to the arptimer() and
  nd6_llinfo_timer() functions with a KASSERT().
  Note: there is no need to return after panic.

  In the legacy IP case, only assign the arg after the check,
  in the IPv6 case, remove the extra checks for the table and
  interface as they have to be there unless we freed and forgot
  to cancel the timer.  It doesn't matter anyway as we would
  panic on the NULL pointer deref immediately and the bug is
  elsewhere.
  This unifies the code of both address families to some extend.

  Reviewed by:  rwatson

git-svn-id: svn://svn.freebsd.org/base/stable/8@210928 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f