On PPC64, when we prepare segments below 4G for use with hugepages, we
need to flush their entries from the SLB, in case SLB entries
specifying normal pages were already present.

Previously we did that by flushing the entire SLB, the patch below
changes this to individually flush each necessary segment with slbie.
The new version may well be slightly faster, but the real reason for
it is so that this code path doesn't need to be changed to reinstate
any bolted SLB entries, if we add them.  The existing version has
already caused problems (read, crashes) when combined with some
patches that add bolted SLB entries.