zram: simplify and optimize dev_to_zram()

Simplify and optimize dev_to_zram() without walking the zram_devices
array.

Change-Id: I7d2031baabfd880ee8479abf3657358cd7db57cb
Signed-off-by: Jiang Liu <jiang.liu@huawei.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Git-commit: 80de574dca050b734d8413a98a983fba3d06240b
Git-repo: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git


Signed-off-by: Olav Haugan <ohaugan@codeaurora.org>