sched: cfs core, kernel/sched_fair.c

add kernel/sched_fair.c - which implements the bulk of CFS's
behavioral changes for SCHED_OTHER tasks.

see Documentation/sched-design-CFS.txt about details.

Authors:

 Ingo Molnar <mingo@elte.hu>
 Dmitry Adamushko <dmitry.adamushko@gmail.com>
 Srivatsa Vaddagiri <vatsa@linux.vnet.ibm.com>
 Mike Galbraith <efault@gmx.de>

Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Mike Galbraith <efault@gmx.de>
Signed-off-by: Dmitry Adamushko <dmitry.adamushko@gmail.com>
Signed-off-by: Srivatsa Vaddagiri <vatsa@linux.vnet.ibm.com>