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

On PPC64, to deal with the restrictions imposed by the PPC MMU's segment
design, hugepages are only allowed to be mapping in two fixed address
ranges, one 2-3G (for use by 32-bit processes) and one 1-1.5T (for use in
64-bit processes).  This is quite limiting, particularly for 32-bit
processes which want to use a lot of large page memory.

This patch relaxes this restriction, and allows any of the low 16 segments
(i.e.  those below 4G) to be individually switched over to allow hugepage
mappings (provided the segment does not already have any normal page
mappings).  The 1-1.5T fixed range for 64-bit processes remains.