kernel - Fix namecache race & panic

kernel - Fix namecache race & panic

* Properly lock and re-check the parent association when iterating its
  children, fixing a bug in a code path associated with unmounting
  filesystems.

  The code improperly assumed that there could be no races because there
  are were no accessors left.  In fact, under heavy loads, the namecache
  scan in this routine can race against the negative-name-cache management
  code.

* Generally speaking can only happen when lots of mounts and unmounts are
  done under heavy loads (for example, tmpfs mounts during a poudriere or
  synth run).