datapath: Don't free vport until all references are gone.

datapath: Don't free vport until all references are gone.

We currently call vport_free() for internal devices after the
device is unregistered.  This takes care of callers that use
either RTNL or RCU but not ones that have only a device reference.
In particular, if stats are requested while a datapath is being
unregistered we can try to use the vport data structures which
have already been freed.

Bug #4736

Reported-by: Brad Hall <brad@nicira.com>
Signed-off-by: Jesse Gross <jesse@nicira.com>
Acked-by: Ben Pfaff <blp@nicira.com>