mm/zsmalloc: simplify shrinker init/destroy

Structure zs_pool has special flag to indicate success of shrinker
initialization.  unregister_shrinker() has improved and can detect by
itself whether actual deinitialization should be performed or not, so
extra flag becomes redundant.

[akpm@linux-foundation.org: update comment (Aliaksei), remove unneeded cast]
Link: http://lkml.kernel.org/r/1513680552-9798-1-git-send-email-akaraliou.dev@gmail.com
Signed-off-by: Aliaksei Karaliou <akaraliou.dev@gmail.com>
Reviewed-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Acked-by: Minchan Kim <minchan@kernel.org>
Cc: Michal Hocko <mhocko@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>