xfs: add a proper transaction pointer to struct xfs_buf

xfs: add a proper transaction pointer to struct xfs_buf

Source kernel commit bf9d9013a2a559858efb590bf922377be9d6d969

Replace the typeless b_fspriv2 and the ugly macros around it with a properly
typed transaction pointer.  As a fallout the log buffer state debug checks
are also removed.  We could have kept them using casts, but as they do
not have a real purpose we can as well just remove them.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Alex Elder <aelder@sgi.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>