Personally, I think if len>MHLEN, m_pullup should create a
  clustered mbuf and return that.  *I* shouldn't have to care
  about what type the mbuf currently is, that's the business of
  the internals of the mbuf API.

Yes, except that this has resource implications - someone might want
to say "if pullup is trivial, do it, and if it's hard, fail".  So
perhaps m_pullup_c, which does as you suggest, and m_pullup which
still fails if things don't fit.  Arguably this iisn't necessary since
the caller knows the length, but MHLEN can change.

-- 
        Greg Troxel <gdt@ir.bbn.com>