hugetlb: multiple hstates for multiple page sizes

hugetlb: multiple hstates for multiple page sizes

Add basic support for more than one hstate in hugetlbfs.  This is the key
to supporting multiple hugetlbfs page sizes at once.

- Rather than a single hstate, we now have an array, with an iterator
- default_hstate continues to be the struct hstate which we use by default
- Add functions for architectures to register new hstates

[[email protected]: coding-style fixes]
Acked-by: Adam Litke <[email protected]>
Acked-by: Nishanth Aravamudan <[email protected]>
Signed-off-by: Andi Kleen <[email protected]>
Signed-off-by: Nick Piggin <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>