xfs_qm_dqflock_pushbuf_wait() does a very similar trick to item
pushing used to do to flush out delayed write dquot buffers. Change
it to use the new promotion method rather than an async flush.

Also, xfs_qm_dqflock_pushbuf_wait() can return without the flush lock
held, yet the callers make the assumption that after this call the
flush lock is held. Always return with the flush lock held.
Signed-off-by: NDave Chinner <david@fromorbit.com>
Reviewed-by: NChristoph Hellwig <hch@lst.de>