network - Increase basic mbuf size from 256 to 384 bytes

network - Increase basic mbuf size from 256 to 384 bytes

* Due to the bloat in m_hdr and m_pkthdr the 256-byte mbuf structure
  is no longer large enough and there appears to be quite a bit of
  legacy code still using m_get() and making assumptions on the
  available space without checking actual space.

  We have assertions in place to catch these but stabilizing the
  system is more important right now.

* Increase the basic mbuf buffer size from 256 to 384 bytes.