Acquire reference to interface and
exit net_epoch earlier in ifmcstat sysctl.
IMPORTANT:
This patch will not completely fix the underlying problem, and
I don't consider it an appropriate long-term solution.
The real fix is to migrate to netlink instead of using sysctl and to avoid
wiring user memory while holding various locks (including vslock). That will
take time.
I am proposing this as a short-term remedy in response to zlei's email to reduce
amount of witness warning.
Personally, I prefer to wait until netlink migration is complete.