xfs: rewrite xfs_bmap_first_unused to make better use of xfs_iext_get_extent

Look at the return value of xfs_iext_get_extent instead of figuring out
the extent count first and looping up to it.

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>