MFC r255111, r255112, r255131, r255167, r256066, r261150, r261151,

MFC r255111, r255112, r255131, r255167, r256066, r261150, r261151,
    r261164, r261166, r261167, r261168, r261394, r261395:

This updates the network driver to support multiple queues, and several bug
fixes. Note that multiqueue support is not compiled in by default since that
would change ALTQ behavior.

  - Sync VirtIO net device header file from recent Linux
  - Import multiqueue VirtIO net driver
  - Fix build with gcc
  - Do not hold the vtnet Rx queue lock when calling up into the stack
  - Read and write the MAC address in the config space byte by byte
  - Also include the mbuf's csum_flags in an assert message
  - Remove stray space
  - Move duplicated transmit start code into a single function
  - Avoid queue unlock followed by relock when the enable interrupt race is
    lost
  - Check for a full virtqueue in the multiqueue transmit path
  - Do not place the sglist used for Rx/Tx on the stack
  - Use m_defrag() instead of m_collapse() to compact a long mbuf chain

git-svn-id: svn://svn.freebsd.org/base/stable/9@265480 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f