mm: preemptibility -v2

Hi,

This (still incomplete) patch-set makes part of the mm a lot more preemptible.
It converts i_mmap_lock and anon_vma->lock to mutexes.  On the way there it
also makes mmu_gather preemptible.

The main motivation was making mm_take_all_locks() preemptible, since it
appears people are nesting hundreds of spinlocks there.

The side-effects are that we can finally make mmu_gather preemptible, something
which lots of people have wanted to do for a long time.

It also gets us anon_vma refcounting which seems to be wanted by KSM as well as
Mel's compaction work.

This patch-set seems to build and boot on my x86_64 machines and even builds a
kernel. I've also attempted powerpc and sparc, which I've compile tested with
their respective defconfigs, remaining are (afaikt the rest uses the generic
tlb bits):

 - s390
 - ia64
 - arm
 - superh
 - um

From those, s390 and ia64 look 'interesting', arm and superh seem very similar
and should be relatively easy (-rt has a patchlet for arm iirc).

What kind of performance tests would people have me run on this to satisfy
their need for numbers? I've done a kernel build on x86_64 and if anything that
was slightly faster with these patches, but it was well within the noise
levels so it might be heat noise I'm looking at ;-)



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