This is a preparation patch for stats code duplication removal.

1) use atomic64_t for `pages_zero' and `pages_stored' zram stats.

2) `compr_size' and `pages_zero' struct zram_stats members did not
   follow the existing device attr naming scheme: zram_stats.ATTR has
   ATTR_show() function.  rename them:

   -- compr_size -> compr_data_size
   -- pages_zero -> zero_pages

Minchan Kim's note:
 If we really have trouble with atomic stat operation, we could
 change it with percpu_counter so that it could solve atomic overhead and
 unnecessary memory space by introducing unsigned long instead of 64bit
 atomic_t.
Signed-off-by: NSergey Senozhatsky <sergey.senozhatsky@gmail.com>
Acked-by: NMinchan Kim <minchan@kernel.org>
Acked-by: NJerome Marchand <jmarchan@redhat.com>
Cc: Nitin Gupta <ngupta@vflare.org>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>