bonding: consolidate ASSERT_RTNL()s and remove the unnecessary

Consolidate the calls to ASSERT_RTNL() before bond_select_active_slave()
inside bond_select_active_slave() itself and remove the ASSERT_RTNL()
from bond_hw_addr_swap() as it's not exported and its only caller -
bond_change_active_slave() already has an ASSERT_RTNL().

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