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.