xfs: separate dquot buffer reads from xfs_dqflush

The first step towards holding the dquot buffer in the li_buf instead of
reading it in the AIL is to separate the part that reads the buffer from
the actual flush code.  There should be no functional changes.

Signed-off-by: "Darrick J. Wong" <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>