mm, highmem: makes flush_all_zero_pkmaps() return index of last flushed entry

mm, highmem: makes flush_all_zero_pkmaps() return index of last flushed entry

In current code, after flush_all_zero_pkmaps() is invoked we re-iterate
all pkmaps.  This can be optimized if flush_all_zero_pkmaps() returns an
index of flushed entry.  With this index, we can immediately map highmem
page to virtual address represented by index.  So change return type of
flush_all_zero_pkmaps() and return index of last flushed entry.

Additionally, update last_pkmap_nr to this index.  It is certain that
entry which is below this index is occupied by other mapping, therefore
updating last_pkmap_nr to this index is reasonable optimization.

Signed-off-by: Joonsoo Kim <js1304@gmail.com>
Cc: Mel Gorman <mel@csn.ul.ie>
Cc: Minchan Kim <minchan@kernel.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>