bonding: call slave_last_rx() only once per slave

Simple cleanup to not call slave_last_rx() on every time function. It won't
give any measurable boost - but looks cleaner and easier to understand.

There are no time-consuming functions in between these calls, so it's safe
to call it in the beginning only once.

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>