xfs: clean up dqblk extraction

Since the introduction of xfs_dqblk in V5, xfs really ought to find the
dqblk pointer from the dquot buffer, then compute the xfs_disk_dquot
pointer from the dqblk pointer.  Fix the open-coded xfs_buf_offset calls
and do the type checking in the correct order.

Note that this has made no practical difference since the start of the
xfs_disk_dquot is coincident with the start of the xfs_dqblk.

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