Revision 306685

Merge r306212:
  Fix regression from r297400, which truncates headers in case of low socket
  buffer and put a small optimization for low socket buffer case:

  - Do not hack uio_resid, and let m_uiotombuf() properly take care of it. This
    fixes truncation of headers at low buffer.
  - If headers ate all the space, jump right to the end of the cycle, to
    avoid doing single page I/O and allocating zero length mbuf.
  - Clear hdr_uio only if space is positive, which indicates that all uio
    was copied in.