xfs: refactor xfs_dabuf_map

Merge xfs_buf_map_from_irec and xfs_da_map_covers_blocks into a single
loop in the caller.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>