IPoIB: Consolidate rtnl_lock tasks in workqueue

Setting the MTU can safely be moved to the carrier_on_task, which keeps
us from needing to take the rtnl lock in the join_finish section.

Signed-off-by: Doug Ledford <dledford@redhat.com>
Signed-off-by: Roland Dreier <roland@purestorage.com>