datapath: Don't dereference potentially NULL vport in patch ports.

datapath: Don't dereference potentially NULL vport in patch ports.

When changing patch ports we need to update any peers pointing to us.
When destroying the port, this means pointing them to a NULL port.
However, the new namespaces code attempted to dereference this port
in order to get the namespace.  The net is already passed in, so we
can just use that instead.

Bug #9477

Reported-by: Michael Hu <mhu@nicira.com>
Signed-off-by: Jesse Gross <jesse@nicira.com>
Acked-by: Ben Pfaff <blp@nicira.com>