As pointed out by Johannes -- the PG prefix seems to stand for page, and
all stat names that contain it represent some per-page event. PGSTALL is
not a page event. This patch renames it.
This is a fix for the mmotm patch
mm-vmstat-account-per-zone-stalls-and-pages-skipped-during-reclaim.patch
Signed-off-by: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>