mm/vmscan: rename zone_nr_pages() to zone_nr_lru_pages()

The name `zone_nr_pages' can be mis-read as zone's (total) number pages,
but it actually returns zone's LRU list number pages.

Signed-off-by: Vincent Li <macli@brc.ubc.ca>
Cc: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Cc: Rik van Riel <riel@redhat.com>
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>