UPSTREAM: zram: use proper type to update max_used_pages

(cherry-pick from commit 2ea55a2caee016daee511431217861fb04767b27)

max_used_pages is defined as atomic_long_t so we need to use unsigned
long to keep temporary value for it rather than int which is smaller
than unsigned long in a 64 bit system.

Bug: 25951511

Change-Id: Id96b1d5a4e033931354eea38ca5e8f3ebd41330d
Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Cc: Minchan Kim <minchan@kernel.org>
Cc: Jerome Marchand <jmarchan@redhat.com>
Cc: Nitin Gupta <ngupta@vflare.org>
Cc: Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>