[RFC] hugetlb reservations -- MAP_PRIVATE fixes for split vmas V2

[Andy Whitcroft]

As reported by Adam Litke and Jon Tollefson one of the libhugetlbfs
regression tests triggers a negative overall reservation count.  When
this occurs where there is no dynamic pool enabled tests will fail.

Following this email are two patches to address this issue:

hugetlb reservations: move region tracking earlier -- simply moves the
  region tracking code earlier so we do not have to supply prototypes, and

hugetlb reservations: fix hugetlb MAP_PRIVATE reservations across vma
  splits -- which moves us to tracking the consumed reservation so that
  we can correctly calculate the remaining reservations at vma close time.

This stack is against the top of v2.6.25-rc6-mm3, should this solution
prove acceptable it would need slipping underneath Nick's multiple hugepage
size patches and those updated.  I have a modified stack prepared for that.

This version incorporates Mel's feedback (both cosmetic, and an allocation
under spinlock issue) and has an improved layout.

Changes in V2:
 - commentry updates
 - pull allocations out from under hugetlb_lock
 - refactor to match shared code layout
 - reinstate BUG_ON's

Jon could you have a test on this and see if it works out for you.

-apw
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