[RFC] Convert fence to use 64bits sequence

[j.glisse at gmail.com]

So this patchset convert the fence to use 64bits sequence and simplify
the fence code (dropping fence lock). I am still convinced that the
best solution is to have the various helper code deals with fence
cleanup/processing. The last patch show an example of what can be
done to improve sa allocator taking advantage of u64 fence.

Cheers,
Jerome