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>

Documentation/blockdev/zram.txt

@@ -169,6 +169,7 @@ comp_algorithm    RW    show and change the compression algorithm
 compact           WO    trigger memory compaction
 debug_stat        RO    this file is used for zram debugging purposes
 backing_dev	  RW	set up backend storage for zram to write out
+idle		  WO	mark allocated slot as idle
 
 
 User space is advised to use the following files to read the device statistics.
@@ -251,16 +252,17 @@ pages of the process with*pagemap.
 If you enable the feature, you could see block state via
 /sys/kernel/debug/zram/zram0/block_state". The output is as follows,
 
-	  300    75.033841 .wh
-	  301    63.806904 s..
-	  302    63.806919 ..h
+	  300    75.033841 .wh.
+	  301    63.806904 s...
+	  302    63.806919 ..hi
 
 First column is zram's block index.
 Second column is access time since the system was booted
 Third column is state of the block.
 (s: same page
 w: written page to backing store
-h: huge page)
+h: huge page
+i: idle page)
 
 First line of above example says 300th block is accessed at 75.033841sec
 and the block's state is huge so it is written back to the backing