- switch to yield function as suggested by you, Arjan and Andrew.
- fixed broken logic in the use of time_before/time_after - possible
bug cause in previous design - in most of the places we were going to sleep
and than check if time expires before checking if condition is satisfied.
If, for example, we needed to wait up to 3 jiffies we could do
schedule_timeout(1) and get up after 4 ticks check that time expired and go
away crying about failure without checking that condition is OK.(in fact I
saw it happen on one SMP platform here).