- Reference count the sub-channel when channel offer message is
processed, so that immediate rescind message on the same channel
will not race sub-channel open on driver side.
- Drop the above reference when sub-channel is closed, this closely
mimics the hypervisor's reaction when primary channel is closed
on the VM side. No drivers use sub-channel after primary channel
is closed.
MFC after: 1 week
Sponsored by: Microsoft
Differential Revision: https://reviews.freebsd.org/D8546
91955558f5