sched/prio: Add 3 macros of MAX_NICE, MIN_NICE and NICE_WIDTH in prio.h

Currently there is lots of hard coding to 19 and -20, to represent
maximum and minimum of nice values.

This patch add three macros in prio.h for maximum, minimum and width
of nice value, and uses it to remove hardcoded values in prio.h.

Signed-off-by: Dongsheng Yang <yangds.fnst@cn.fujitsu.com>
Signed-off-by: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/3994e89327b2b15f992277cdf9f409c516f87d1b.1392103744.git.yangds.fnst@cn.fujitsu.com


Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
[ Collapsed two small patches. ]
Signed-off-by: Ingo Molnar <mingo@kernel.org>