The struct xfs_dabuf now only tracks a single xfs_buf and all the
information it holds can be gained directly from the xfs_buf. Hence
we can remove the struct dabuf and pass the xfs_buf around
everywhere.

Kill the struct dabuf and the associated infrastructure.
Signed-off-by: NDave Chinner <dchinner@redhat.com>
Reviewed-by: NChristoph Hellwig <hch@lst.de>
Signed-off-by: NBen Myers <bpm@sgi.com>