Use kernel standard bit spin-lock instead of custom mess.  Even, it has
a bug which doesn't disable preemption.  The reason we don't have any
problem is that we have used it during preemption disable section by
class->lock spinlock.  So no need to go to stable.

Link: http://lkml.kernel.org/r/1464736881-24886-6-git-send-email-minchan@kernel.orgSigned-off-by: NMinchan Kim <minchan@kernel.org>
Reviewed-by: NSergey Senozhatsky <sergey.senozhatsky@gmail.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>