xen-netback: keep a local pointer for vif in backend_disconnect()

This patch replaces use of 'be->vif' with 'vif' and hence generally
makes the function look tidier. No semantic change.

Signed-off-by: Paul Durrant <paul.durrant@citrix.com>
Signed-off-by: David S. Miller <davem@davemloft.net>