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SubjectSCHED_DEADLINE cpudeadline.{h,c} fixup

this is a rework of the cpudeadline bugfix and speed-up patch-set, that
integrates all comments received so far from Luca and Juri.

The first patch is a minimally invasive (1-line) fix for the deadline
wrap-around bug. This leaves some weirdness in how cpudl_change_key() is
called. Therefore, the second patch does a minimum of refactory to make
things more explicit and clear.
The 3rd patch contains now the actual performance enhancement (avoiding
unneeded swaps during heapify operations), which has been measured to
achieve up to 10% , of speed-up for cpudl_set() calls.
This has been measured with a andomly generated workload of 1K,10K,100K
random heap insertions and deletions (75% cpudl_set() calls with is_valid=1
and 25% with is_valid=0), and randomly generated cpu IDs, with up to 256

Benchmarking code is available at:
Obtained speed-up plot:

Finally, the 4th patch is another clear-up patch touching cpudeadline.{h,c}
and deadline.c. Now you call cpudl_clear(cp, cpu) and cpudl_set(cp, cpu, dl)
instead of cpudl_set(cp, cpu, 0 /* dl */, 0 /* is_valid */) and
cpudl_set(cp, cpu, dl, 1 /* is_valid */).

Please, share your comments, thanks!


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