bonding: rework bond_find_best_slave() to use bond_for_each_slave()

bond_find_best_slave() does not have to be balanced - i.e. return the slave
that is *after* some other slave, but rather return the best slave that
suits, except of bond->primary_slave - in which case we just return it if
it's suitable.

After that we just look through all the slaves and return either first up
slave or the slave whose link came back earliest.

We also don't care about curr_active_slave lock cause we use it in
bond_should_change_active() only and there we take it right away - i.e. it
won't go away.

CC: Jay Vosburgh <fubar@us.ibm.com>
CC: Andy Gospodarek <andy@greyhouse.net>
Signed-off-by: Veaceslav Falico <vfalico@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>