ocfs2: add kobject for online file check

ocfs2: add kobject for online file check

Use embedded kobject mechanism for online file check feature, this will
avoid to use a global list to save/search per-device online file check
related data, meanwhile, reduce the code lines and make the code logic
clear.  The changed code is based on Goldwyn Rodrigues's patches and
ext4 fs code.

Link: http://lkml.kernel.org/r/1495611866-27360-4-git-send-email-ghe@suse.com
Signed-off-by: Gang He <ghe@suse.com>
Cc: Mark Fasheh <mark@fasheh.com>
Cc: Joel Becker <jlbec@evilplan.org>
Cc: Junxiao Bi <junxiao.bi@oracle.com>
Cc: Joseph Qi <jiangqi903@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>