[POWERPC] powerpc: Fix demotion of segments to 4K pages

[POWERPC] powerpc: Fix demotion of segments to 4K pages

When demoting a process to use 4K HW pages (instead of 64K), which
happens under various circumstances such as doing cache inhibited
mappings on machines that do not support 64K CI pages, the assembly
hash code calls back into the C function flush_hash_page().  This
function prototype was recently changed to accomodate for 1T segments
but the assembly call site was not updated, causing applications that
do demotion to hang.  In addition, when updating the per-CPU PACA for
the new sizes, we didn't properly update the slice "map", thus causing
the SLB miss code to re-insert segments for the wrong size.

This fixes both and adds a warning comment next to the C
implementation to try to avoid problems next time someone changes it.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>