zram: rework reset and destroy path

We need to return set_capacity(disk, 0) from reset_store() back to
zram_reset_device(), a catch by Ganesh Mahendran.  Potentially, we can
race set_capacity() calls from init and reset paths.

The problem is that zram_reset_device() is also getting called from
zram_exit(), which performs operations in misleading reversed order -- we
first create_device() and then init it, while zram_exit() perform
destroy_device() first and then does zram_reset_device().  This is done to
remove sysfs group before we reset device, so we can continue with device
reset/destruction not being raced by sysfs attr write (f.e.  disksize).

Apart from that, destroy_device() releases zram->disk (but we still have
->disk pointer), so we cannot acces zram->disk in later
zram_reset_device() call, which may cause additional errors in the future.

So, this patch rework and cleanup destroy path.

1) remove several unneeded goto labels in zram_init()

2) factor out zram_init() error path and zram_exit() into
   destroy_devices() function, which takes the number of devices to
   destroy as its argument.

3) remove sysfs group in destroy_devices() first, so we can reorder
   operations -- reset device (as expected) goes before disk destroy and
   queue cleanup.  So we can always access ->disk in zram_reset_device().

4) and, finally, return set_capacity() back under ->init_lock.

[akpm@linux-foundation.org: tweak comment]
Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Reported-by: Ganesh Mahendran <opensource.ganesh@gmail.com>
Cc: Minchan Kim <minchan@kernel.org>
Cc: Jerome Marchand <jmarchan@redhat.com>
Cc: Nitin Gupta <ngupta@vflare.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>