MFC r285973

MFC r285973
    - Avoid lock contention in the if_transmit callback by using trylock and
     enqueueing the frames when it fails. This way there is some latency
     removed from the transmitting path.
    - If IFF_DRV_OACTIVE is set (and also if IFF_DRV_RUNNING is not) just
     enqueue the desired frames and return successful transmit. This way we
     avoid to return errors on transmit side and resulting in
     possible out-of-order frames. Please note that IFF_DRV_OACTIVE is set
     everytime we get the threshold ring hit, so this can be happening quite
     often.

Submitted by:	Attilio.Rao@isilon.com

git-svn-id: svn://svn.freebsd.org/base/stable/8@286300 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f