MFC r259212, r259220:

MFC r259212, r259220:

  Fix one race and one fence post error. When the TX buffer was
  completely full, we'd not complete any of the mbufs due to the fence
  post error (this creates a large leak). When this is fixed, we still
  leak, but at a much smaller rate due to a race between ateintr and
  atestart_locked as well as an asymmetry where atestart_locked is
  called from elsewhere.  Ensure that we free in-flight packets that
  have completed there as well. Also remove needless check for NULL on
  mb, checked earlier in the loop and simplify a redundant if.

git-svn-id: svn://svn.freebsd.org/base/stable/10@259381 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f