xfs: stop using xfs_qm_dqtobp in xfs_qm_dqflush

xfs: stop using xfs_qm_dqtobp in xfs_qm_dqflush

In xfs_qm_dqflush we know that q_blkno must be initialized already from a
previous xfs_qm_dqread.  So instead of calling xfs_qm_dqtobp we can
simply read the quota buffer directly.  This also saves us from a duplicate
xfs_qm_dqcheck call check and allows xfs_qm_dqtobp to be simplified now
that it is always called for a newly initialized inode.  In addition to
that properly unwind all locks in xfs_qm_dqflush when xfs_qm_dqcheck
fails.

This mirrors a similar cleanup in the inode lookup done earlier.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Alex Elder <aelder@sgi.com>