MFC 307838,307839

MFC 307838,307839

307838
    hyperv/hn: Move chimney buffer index and size to txdesc.

    All RNDIS control messages have used SG list for a while.  This makes
    the send context suitable for further refactoring.

    Sponsored by:   Microsoft
    Differential Revision:  https://reviews.freebsd.org/D8308

307839
    hyperv/hn: Fix chimney sending buffer leakage upon NVS sending failure.

    This will not happen in real world, since TX consumption of the vmbus
    TX bufring is limitted.  Better safe than sorry.

    Sponsored by:   Microsoft
    Differential Revision:  https://reviews.freebsd.org/D8309

git-svn-id: svn://svn.freebsd.org/base/stable/10@308495 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f