sched: Move the priority specific bits into a new header file

Some bits about priority are defined in linux/sched/rt.h, but
some of them are not only for rt scheduler, such as MAX_PRIO.

This patch move them all into a new header file, linux/sched/prio.h.

Signed-off-by: Dongsheng Yang <yangds.fnst@cn.fujitsu.com>
Cc: clark.williams@gmail.com
Cc: rostedt@goodmis.org
Cc: raistlin@linux.it
Cc: juri.lelli@gmail.com
Signed-off-by: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/f7549508a1588da2c613d601748ca9de30fa5dcf.1390859827.git.yangds.fnst@cn.fujitsu.com


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