zram: count same page write as page_stored

Regardless of whether it is same page or not, it's surely write and
stored to zram so we should increase pages_stored stat.  Otherwise, user
can see zero value via mm_stats although he writes a lot of pages to
zram.

Link: http://lkml.kernel.org/r/1494834068-27004-1-git-send-email-minchan@kernel.org


Signed-off-by: Minchan Kim <minchan@kernel.org>
Reviewed-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>