sched/fair: Introduce and use the vruntime_cmp() and vruntime_op() wrappers for wrapp...

sched/fair: Introduce and use the vruntime_cmp() and vruntime_op() wrappers for wrapped-signed aritmetics

We have to be careful with vruntime comparisons and subtraction,
due to the possibility of wrapping, so we have macros like:

   #define vruntime_gt(field, lse, rse) ({ (s64)((lse)->field - (rse)->field) > 0; })

Which is used like this:

		if (vruntime_gt(min_vruntime, se, rse))
			se->min_vruntime = rse->min_vruntime;

Replace this with an easier to read pattern that uses the regular
arithmetics operators:

		if (vruntime_cmp(se->min_vruntime, ">", rse->min_vruntime))
			se->min_vruntime = rse->min_vruntime;

Also replace vruntime subtractions with vruntime_op():

	-       delta = (s64)(sea->vruntime - seb->vruntime) +
	-               (s64)(cfs_rqb->zero_vruntime_fi - cfs_rqa->zero_vruntime_fi);
	+       delta = vruntime_op(sea->vruntime, "-", seb->vruntime) +
	+               vruntime_op(cfs_rqb->zero_vruntime_fi, "-", cfs_rqa->zero_vruntime_fi);

In the vruntime_cmp() and vruntime_op() macros use Use __builtin_strcmp(),
because of __HAVE_ARCH_STRCMP might turn off the compiler optimizations
we rely on here to catch usage bugs.

No change in functionality.

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