ocfs2: implement delayed dropping of last dquot reference

We cannot drop last dquot reference from downconvert thread as that
creates the following deadlock:

NODE 1                                  NODE2
holds dentry lock for 'foo'
holds inode lock for GLOBAL_BITMAP_SYSTEM_INODE
                                        dquot_initialize(bar)
                                          ocfs2_dquot_acquire()
                                            ocfs2_inode_lock(USER_QUOTA_SYSTEM_INODE)
                                            ...
downconvert thread (triggered from another
node or a different process from NODE2)
  ocfs2_dentry_post_unlock()
    ...
    iput(foo)
      ocfs2_evict_inode(foo)
        ocfs2_clear_inode(foo)
          dquot_drop(inode)
            ...
	    ocfs2_dquot_release()
              ocfs2_inode_lock(USER_QUOTA_SYSTEM_INODE)
               - blocks
                                            finds we need more space in
                                            quota file
                                            ...
                                            ocfs2_extend_no_holes()
                                              ocfs2_inode_lock(GLOBAL_BITMAP_SYSTEM_INODE)
                                                - deadlocks waiting for
                                                  downconvert thread

We solve the problem by postponing dropping of the last dquot reference to
a workqueue if it happens from the downconvert thread.

Signed-off-by: Jan Kara <jack@suse.cz>
Reviewed-by: Mark Fasheh <mfasheh@suse.de>
Reviewed-by: Srinivas Eeda <srinivas.eeda@oracle.com>
Cc: Joel Becker <jlbec@evilplan.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>