xfs: Remove macro XFS_BUF_SET_START

Remove the definition and usage of the macro XFS_BUF_SET_START.

Signed-off-by: Chandra Seetharaman <sekharan@us.ibm.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Alex Elder <aelder@sgi.com>