sched: Limit the scope of clear_buddies

The clear_buddies function does not seem to play well with the concept
of hierarchical runqueues.  In the following tree, task groups are
represented by 'G', tasks by 'T', next by 'n' and last by 'l'.

     (nl)
    /    \
   G(nl)  G
   / \     \
 T(l) T(n)  T

This situation can arise when a task is woken up T(n), and the previously
running task T(l) is marked last.

When clear_buddies is called from either T(l) or T(n), the next and last
buddies of the group G(nl) will be cleared.  This is not the desired
result, since we would like to be able to find the other type of buddy
in many cases.

This especially a worry when implementing yield_task_fair through the
buddy system.

The fix is simple: only clear the buddy type that the task itself
is indicated to be.  As an added bonus, we stop walking up the tree
when the buddy has already been cleared or pointed elsewhere.

Signed-off-by: Rik van Riel <riel@redhat.coM>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
LKML-Reference: <20110201094837.6b0962a9@annuminas.surriel.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>