bonding: consolidate the two rlb_next_rx_slave functions into one

__rlb_next_rx_slave() is a copy of rlb_next_rx_slave() with the
difference that it uses rcu primitives to walk the slave list. We don't
need the two functions and can make rlb_next_rx_slave() a wrapper for
callers which hold RTNL.
So add a comment and ASSERT_RTNL() to make sure what is intended.

Signed-off-by: Nikolay Aleksandrov <nikolay@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>