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

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