zsmalloc: fix zs_init cpu notifier error handling

zsmalloc: fix zs_init cpu notifier error handling

Mahendran Ganesh reported that zpool-enabled zsmalloc should not call
zpool_unregister_driver() from zs_init() if cpu notifier registration has
failed, because error handling is performed before we register the driver
via zpool_register_driver() call.

Factor out cpu notifier registration and unregistration code and fix
zs_init() error handling.

link: http://lkml.iu.edu//hypermail/linux/kernel/1411.1/04156.html
[[email protected]: squash bogus gcc warning]
[[email protected]: use __init and __exit]
Signed-off-by: Sergey Senozhatsky <[email protected]>
Reported-by: Mahendran Ganesh <[email protected]>
Cc: Minchan Kim <[email protected]>
Cc: Nitin Gupta <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>