zram: clear disk io accounting when reset zram device

Clear zram disk io accounting when resetting the zram device.  Otherwise
the residual io accounting stat will affect the diskstat in the next
zram active cycle.

Signed-off-by: Weijie Yang <weijie.yang@samsung.com>
Acked-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Acked-by: Minchan Kim <minchan@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>