zram: add dynamic device add/remove functionality

We currently don't support on-demand device creation.  The one and only
way to have N zram devices is to specify num_devices module parameter
(default value: 1).  IOW if, for some reason, at some point, user wants
to have N + 1 devies he/she must umount all the existing devices, unload
the module, load the module passing num_devices equals to N + 1.  And do
this again, if needed.

This patch introduces zram control sysfs class, which has two sysfs
attrs:
- hot_add      -- add a new zram device
- hot_remove   -- remove a specific (device_id) zram device

hot_add sysfs attr is read-only and has only automatic device id
assignment mode (as requested by Minchan Kim).  read operation performed
on this attr creates a new zram device and returns back its device_id or
error status.

Usage example:
	# add a new specific zram device
	cat /sys/class/zram-control/hot_add
	2

	# remove a specific zram device
	echo 4 > /sys/class/zram-control/hot_remove

Returning zram_add() error code back to user (-ENOMEM in this case)

	cat /sys/class/zram-control/hot_add
	cat: /sys/class/zram-control/hot_add: Cannot allocate memory

NOTE, there might be users who already depend on the fact that at least
zram0 device gets always created by zram_init(). Preserve this behavior.

[minchan@kernel.org: use zram->claim to avoid lockdep splat]
Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Cc: Minchan Kim <minchan@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Documentation/ABI/testing/sysfs-class-zram

@@ -0,0 +1,24 @@
+What:		/sys/class/zram-control/
+Date:		August 2015
+KernelVersion:	4.2
+Contact:	Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
+Description:
+		The zram-control/ class sub-directory belongs to zram
+		device class
+
+What:		/sys/class/zram-control/hot_add
+Date:		August 2015
+KernelVersion:	4.2
+Contact:	Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
+Description:
+		RO attribute. Read operation will cause zram to add a new
+		device and return its device id back to user (so one can
+		use /dev/zram<id>), or error code.
+
+What:		/sys/class/zram-control/hot_remove
+Date:		August 2015
+KernelVersion:	4.2
+Contact:	Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
+Description:
+		WO attribute. Remove a specific /dev/zramX device, where X
+		is a device_id provided by user.