Revert "bridge: Use hlist_for_each_entry_rcu() in br_multicast_add_router()"

This reverts commit ff65e827.

As explained by Stephen Hemminger, the traversal doesn't require
RCU handling as we hold a lock.

The list addition et al. calls, on the other hand, do.

Signed-off-by: David S. Miller <davem@davemloft.net>