Staging: zram: work around oops due to startup ordering snafu

I'm getting an oops when running mkfs on zram:

NIP [d0000000030e0340] .zram_inc_stat+0x58/0x84 [zram]
[c00000006d58f720] [d0000000030e091c] .zram_make_request+0xa8/0x6a0 [zram]
[c00000006d58f840] [c00000000035795c] .generic_make_request+0x390/0x434
[c00000006d58f950] [c000000000357b14] .submit_bio+0x114/0x140
[c00000006d58fa20] [c000000000361778] .blkdev_issue_discard+0x1ac/0x250
[c00000006d58fb10] [c000000000361f68] .blkdev_ioctl+0x358/0x7fc
[c00000006d58fbd0] [c0000000001c1c1c] .block_ioctl+0x6c/0x90
[c00000006d58fc70] [c0000000001984c4] .do_vfs_ioctl+0x660/0x6d4
[c00000006d58fd70] [c0000000001985a0] .SyS_ioctl+0x68/0xb0

Since disksize no longer starts as 0 it looks like we can call
zram_make_request before the device has been initialised. The patch below
fixes the immediate problem but this would go away if we move the
initialisation function elsewhere (as suggested in another thread).

Signed-off-by: Anton Blanchard <anton@samba.org>
Cc: Nitin Gupta <ngupta@vflare.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>