zram: use idr instead of `zram_devices' array

This patch makes some preparations for on-demand device add/remove
functionality.

Remove `zram_devices' array and switch to id-to-pointer translation (idr).
idr doesn't bloat zram struct with additional members, f.e.  list_head,
yet still provides ability to match the device_id with the device pointer.

No user-space visible changes.

[Julia.Lawall@lip6.fr: return -ENOMEM when `queue' alloc fails]
Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Reported-by: Julia Lawall <Julia.Lawall@lip6.fr>
Acked-by: Minchan Kim <minchan@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>