xfs: use discontiguous xfs_buf support in dabuf wrappers

First step in converting the directory code to use native
discontiguous buffers and replacing the dabuf construct.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Ben Myers <bpm@sgi.com>