[BRIDGE]: fix race on bridge del if

This fixes the RCU race on bridge delete interface.  Basically,
the network device has to be detached from the bridge in the first
step (pre-RCU), rather than later. At that point, no more bridge traffic
will come in, and the other code will not think that network device
is part of a bridge.

This should also fix the XEN test problems.

Signed-off-by: Stephen Hemminger <shemminger@osdl.org>
Signed-off-by: David S. Miller <davem@davemloft.net>