IPoIB: Use rtnl lock/unlock when changing device flags

Use of this lock is required to synchronize changes to the netdvice's
data structs.  Also move the call to ipoib_flush_paths() after the
modification of the netdevice flags in set_mode().

Signed-off-by: Eli Cohen <eli@mellanox.co.il>
Signed-off-by: Roland Dreier <rolandd@cisco.com>