IPoIB: Avoid free_netdev() BUG when destroying a child interface

We have to release the RTNL before calling free_netdev() so that the
device state has a chance to become NETREG_UNREGISTERED.  Otherwise
when removing a child interface, we hit the BUG() that tests the
device state in free_netdev().

Reported-by: Yossi Etigin <yosefe@voltaire.com>
Signed-off-by: Roland Dreier <rolandd@cisco.com>