MFC r218360 (by luigi):

MFC r218360 (by luigi):
    correct the 'output_time' of packets generated by dummynet.
    In the dec.2009 rewrite I introduced a bug, using for the
    computation the arrival time instead of the time the packet
    has exited from the queue.
    The bandwidth computation was still correct because it is
    computed elsewhere, but traffic was sent out in bursts.

    The bug is also present in RELENG_8 after dec.2009

    Thanks to Daikichi Osuga for investingating, finding and fixing the
    bug with detailed graphs of the behaviour before and after the fix.

    Submitted by:	Daikichi Osuga

git-svn-id: svn://svn.freebsd.org/base/stable/8@221360 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f