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

On PowerPC64 the "low" hugepage range (at 2-3G for use by 32-bit processes)
needs to be activated before it can be used.  hugetlb_get_unmapped_area()
automatically activates the range for hugepage mappings in 32-bit processes
which are not MAP_FIXED.  However for MAP_FIXED mmap()s, even at a suitable
address will fail if the region is not already activated, because there is
no suitable callback from the generic MAP_FIXED code path into the arch
code.

This patch corrects this problem and allows PPC64 to do MAP_FIXED hugepage
mappings in the low hugepage range.