Don't free mbufs when stopping an interface in netmap mode.

Currently if you ifconfig down a vtnet interface while it is being used
via netmap, the kernel panics due to trying to treat the cookie values
in the virtio rings as mbufs to be freed. When netmap is enabled, these
cookie values are pointers to something else.

Note that other netmap-aware drivers don't seem to need this as they
store the mbuf pointers in the software rings that mirror the hardware
descriptor rings, and since netmap doesn't touch those, the software
state always has NULL mbuf pointers causing the loops to free mbufs to
not do anything. However, vtnet reuses the same state area for both
netmap and non-netmap mode, so it needs to explicitly avoid looking at
the rings and treating the cookie values as mbufs if netmap is
enabled.

Differential Revision: https://reviews.freebsd.org/D2348
Reviewed by: adrian, bryanv, luigi
MFC after: 1 week
Sponsored by: Norse Corp, Inc.