zram: introduce ZRAM_IDLE flag

To support idle page writeback with upcoming patches, this patch
introduces a new ZRAM_IDLE flag.

Userspace can mark zram slots as "idle" via
	"echo all > /sys/block/zramX/idle"
which marks every allocated zram slot as ZRAM_IDLE.
User could see it by /sys/kernel/debug/zram/zram0/block_state.

          300    75.033841 ...i
          301    63.806904 s..i
          302    63.806919 ..hi

Once there is IO for the slot, the mark will be disappeared.

	  300    75.033841 ...
          301    63.806904 s..i
          302    63.806919 ..hi

Therefore, 300th block is idle zpage. With this feature,
user can how many zram has idle pages which are waste of memory.

Link: http://lkml.kernel.org/r/20181127055429.251614-5-minchan@kernel.org


Signed-off-by: Minchan Kim <minchan@kernel.org>
Reviewed-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Reviewed-by: Joey Pabalinas <joeypabalinas@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>