vport: Make dp_port->vport always valid.

vport: Make dp_port->vport always valid.

When we detached a vport we would assign NULL to dp_port->vport
before calling synchronize_rcu().  However, since vports have a
longer lifetime than dp_ports there were no checks before
dereferencing dp_port->vport.  This changes the behavior to
match the assumption by not assigning NULL during detach.  This
avoids a potential NULL pointer dereference in do_output() among
other places.