sched: Expose some macros related to priority

Some macros in kernel/sched/sched.h about priority are
private to kernel/sched. But they are useful to other
parts of the core kernel.

This patch moves these macros from kernel/sched/sched.h to
include/linux/sched/prio.h so that they are available to
other subsystems.

Signed-off-by: Dongsheng Yang <yangds.fnst@cn.fujitsu.com>
Cc: raistlin@linux.it
Cc: juri.lelli@gmail.com
Cc: clark.williams@gmail.com
Cc: rostedt@goodmis.org
Signed-off-by: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/2b022810905b52d13238466807f4b2a691577180.1390859827.git.yangds.fnst@cn.fujitsu.com


Signed-off-by: Ingo Molnar <mingo@kernel.org>