ocfs2: fix NULL pointer dereference when dismount and ocfs2rec simultaneously

2 nodes cluster, say Node A and Node B, mount the same ocfs2 volume, and
create a file 1.

Node A			Node B
open 1, get open lock
                        rm 1, and then add 1 to orphan_dir
storage link down,
o2hb_write_timeout
->o2quo_disk_timeout
->emergency_restart
                        at the moment, Node B dismount and do
			ocfs2rec simultaneously
                        1) ocfs2_dismount_volume
			->ocfs2_recovery_exit
			->wait_event(osb->recovery_event)
			->flush_workqueue(ocfs2_wq)
			2) ocfs2rec
			->queue_work(&journal->j_recovery_work)
                        ->ocfs2_recover_orphans
			->ocfs2_commit_truncate
                        ->queue_delayed_work(&osb->osb_truncate_log_wq)

In ocfs2_recovery_exit, it flushes workqueue and then releases system
inodes.  When doing ocfs2rec, it will call ocfs2_flush_truncate_log
which will try to get sys_root_inode, and NULL pointer dereference
occurs.

Signed-off-by: Yiwen Jiang <jiangyiwen@huawei.com>
Signed-off-by: joyce <xuejiufei@huawei.com>
Signed-off-by: Joseph Qi <joseph.qi@huawei.com>
Cc: Joel Becker <jlbec@evilplan.org>
Cc: Mark Fasheh <mfasheh@suse.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>