From: David Gibson <david@gibson.dropbear.id.au>

Converts the mm_context_t on ppc64 to be a struct.  This lets us separate
the low_hpages flag into a separate field rather than folding it into the
actual context id.  That makes things neater, since the flag is
conceptually separate and has, for example, should be propogate across a
fork whereas the context ID obviously isn't.  The mm_context_id is the only
place to put arch-specific information in the mm_struct.

This patch will also make some interesting extensions to the hugepage
support much easier, such as allowing dynamic resizing of the hugepage
address space, or using special pagetables for hugepages.